Why are the requirements for becoming a GM so complicated? Wikipedia's article on chess grandmasters indicates that the requirements to become a GM is complicated, and has always been complicated. For example right now, to become a GM, one needs to have had a rating of at least 2500 at some point in one's career, and one needs &quot;two favorable results (called norms) from a total of at least 27 games in tournaments&quot;, with the rules for what qualifies as &quot;norm&quot; occupying several more bullet points (quotes from the WIkipedia article).
Why does it need to be so? If the aim is to award the title only to world class players, I can think of some much easier system to select them, e.g. you must have been rated in the top 100 of the world at one point. This would tie the title to a single, easily-understood number. I notice some of the lower titles already use this criterion (e.g. Candidate Master), so it begs the question of why the higher ones have more complicated requirements.
If the aim is to award the title only to world class players, I can
think of some much easier system to select them, e.g. you must have
been rated in the top 100 of the world at one point. This would tie
the title to a single, easily-understood number. I notice some of the
lower titles already use this criterion (e.g. Candidate Master)

The comparison with the Candidate Master title is a telling one. The CM title is not highly regarded. This has been covered in two previous questions here - How respectable is the CM title? and Is a candidate master considered a master?

Why are the requirements for becoming a GM so complicated?

They are indeed complicated. Here is the link for the FIDE Title Regulations effective from 1 July 2017. One additional requirement, which I don't see listed here, is that the tournament organizer must send a file with the PGNs of all the games for the tournament.
The reasons they are so complicated is that they have evolved over time to deal with the corrupting influences of money, politics, rating manipulation and cheating.
Money
One of the ways of getting the GM title is to be World Champion. Here is what Wikipedia says about the early period of world championships:

From 1886 to 1946, the champion set the terms, requiring any
challenger to raise a sizable stake and defeat the champion in a match
in order to become the new world champion.

So, no money - no match. As the article says, this had a profound effect on who got to play for the title:

This requirement makes arranging world championship matches more
difficult, for example: Marshall challenged Lasker in 1904 but could
not raise the money until 1907;[37] in 1911 Lasker and Rubinstein
agreed in principle to a world championship match, but this was never
played as Rubinstein could not raise the money.[38][39] In the early
1920s, Alekhine, Rubinstein and Nimzowitsch all challenged Capablanca,
but only Alekhine was able to raise the US$10,000 that Capablanca
demanded, and not until 1927

OK, you might be thinking, those kinds of considerations belong to a bygone age. Well, try telling that to Shirov who, perhaps, should have been World Champion in 2000 instead of Kramnik:

Soon after the 1995 championship, the PCA folded, and Kasparov had no
organisation to choose his next challenger. In 1998 he formed the
World Chess Council, which organised a candidates match between Alexei
Shirov and Vladimir Kramnik. Shirov won the match, but negotiations
for a Kasparov–Shirov match broke down, and Shirov was subsequently
omitted from negotiations, much to his disgust. Plans for a 1999 or
2000 Kasparov–Anand match also broke down, and Kasparov organised a
match with Kramnik in late 2000

Politics
This is what the Wikipedia article has to say on this:

Title awards under the original regulations were subject to political
concerns. Efim Bogoljubov, who had emigrated from the Soviet Union to
Germany, was not entered in the first class of Grandmasters, even
though he had played two matches for the World Championship with
Alekhine. He received the title in 1951, by a vote of thirteen to
eight with five abstentions. Yugoslavia supported his application, but
all other Communist countries opposed it. In 1953, FIDE abolished the
old regulations, although a provision was maintained that allowed
older masters who had been overlooked to be awarded titles

There was more political interference in the 1962 Candidates tournament which selected the challenger for the 1963 World Championships. Here is what the article says:

What makes this tournament famous and often-discussed is the
allegations of Soviet collusion. The three top finishers (Petrosian,
Geller and Keres) drew all twelve of their games against each other,
in an average of only 19 moves.[15]
Soon after the tournament, Fischer publicly alleged that the Soviets
had colluded to prevent any non-Soviet – specifically him – from
winning. His allegations were twofold: first, that Petrosian, Geller
and Keres had pre-arranged to draw all their games; and second, that
Korchnoi had been part of the drawing pact in the first half of the
tournament, and been instructed to lose some games to them in the
second half.[16][17][18] (In the first two cycles Korchnoi drew all
his games with Petrosian, Geller and Keres; in the third cycle he lost
to all of them; and in the final cycle he lost to Petrosian but drew
with Keres and Geller).
The first allegation, of the drawing pact, is generally assumed to be
correct.[18] All of the three players involved have since died, but
Yuri Averbakh, who was head of the Soviet team, confirmed it in a 2002
interview.[10] He offered the rationale that Keres was the oldest
competitor and wanted to conserve energy, and that Petrosian and
Geller were good friends with a history of drawing with each other.
The second allegation, of Korchnoi throwing games, is more
doubtful.[18] Korchnoi defected from the USSR in 1976, and never
alleged he was forced to throw games. Dominic Lawson calls the
allegation &quot;preposterous&quot;, noting that the main beneficiary of
Korchnoi's losses was Petrosian, whom Korchnoi detested

Rating Manipulation and Cheating
There is an interesting Wikipedia article on this. For those who like their chess dripping with blood there is the story of infamous mother-killer Claude Bloodgood:

Bloodgood organized chess games within Powhatan Correctional Center in
Powhatan, Virginia, which were by necessity with fellow inmates.3
Many of these inmates were taught the game by Bloodgood, and thus
began as unrated and inexperienced players. Bloodgood obtained USCF
memberships for them. Some accused Bloodgood, with his intimate
knowledge of the rating system, of rigging their ratings. The
accusation was that he arranged for new prisoners to play rated games
against other prisoners, who would deliberately lose, thus giving the
new inmate an inflated USCF rating. Bloodgood, it is further alleged,
then played rated games against the new highly rated prisoner, and
each time he won, gained a few more rating points. This continued for
several years, and by 1997 his rating rose to 2759, making the
59-year-old Bloodgood the second highest rated player in the nation,
only behind Gata Kamsky.

OK, but that was USCF rating not FIDE. That could never happen with FIDE ratings, surely? And certianly not for titles? Well, check out this article from TWIC (The Week in Chess) from 8th March 1999. Here is an extract:

Respected Grandmaster and chess journalist Ian Rogers also felt moved
to comment on the event in the Canberra Times Chess Column for March
7th. He too was almost breathtaken by the number of norms scored in
the event and the calculated level of planning required for titles and
rating fraud on this scale. I quote, with permission, at length below.
&quot;The Grandmaster tournament which took place in Rangoon in the first
two weeks of February took months of planning but succeeded in
creating six Grandmaster results and eight IM results, an astonishing
success rate. (Two or three title 'norms' are needed for a player to
be awarded the GM or IM title.) Some of those scoring Grandmaster
results had struggled to score 50% in the South-East Asian zonal
tournament only a month earlier! Corruption extended not only to the
tournament itself - with seven highly rated locals losing all their
games to their compatriots - but also to a series of events held in
1998 which provided extremely high world rankings for all those
required to be 'victims' in the February tournament. All seven of the
no-hopers in February were unrated in January 1998, yet on the January
1999 FIDE list had rankings higher than Australian Grandmaster Darryl
Johansen.&quot;
&quot;The following game is not atypical of their play, with the heavily
'outrated' Mas winning one of the easiest games of his career: Rangoon
1999 White: H.Mas Black: Thein Dan Oo 1.e4 c5 2.f4 d6 3.Nf3 Nc6 4.Bb5
g6 5.0-0 Bg7 6.Bxc6+ bxc6 7.d3 Rb8?! 8.Nc3 Nf6 9.Qe1 0-0 10.b3 d5?
11.e5 Nd7 12.Ba3 Re8?! 13.Na4 Rb5 14.e6 Nf6? 15.c4 Ra5? 16.exf7+ Kxf7 17.Ne5+ Kg8 18.Nxc6 1-0 Mas, the honourable Malaysian IM who performed so creditably in Sydney in January, was not one of the favoured many
who scored title results. His opponent, rated 2539 - a near-GM
ranking, managed only 4 points from 14 games. There is no suggestion
that any of the foreign visitors were involved in anything other than
pre-arranged draws - they simply took advantage of the extraordinarily
overrated opposition. However the local players in Rangoon probably
knew the results of many of their games before the event began and if
an 'accident' occurred to one of those destined to score a title norm

e.g. a loss to a foreign player - their compatriots pitched in and provided some extra points or half points. Unusually for a corrupt
tournament, the foreign invitees were not needed to help out the
locals, since the enormous rankings of the weakest players meant that
the scores required for title results were relatively low.&quot;

&quot;From the point of view of the organisers, the event was so successful
that it is likely to be repeated in a few months time, before the
ratings of the tailenders are adjusted on the July FIDE rating list.
After one more such tournament, Burma could have 4 Grandmasters and
half a dozen IMs; after a few more such tournaments they could have
more Grandmasters than any country in Asia! Publicity may encourage
the Burmese to become more discreet in future but FIDE is highly
unlikely to take any action. Their actions in past cases would
indicate that the world body tacitly approves of a 'developing' chess
nation acquiring titled players, whatever the ethics involved.&quot;