Why the initial rating of FIDE is calculated that strangely? Let suppose an unrated player plays a FIDE rated swiss tournament where its opponent's ratings are e.g. 1300,2000,2100,2200,2300 and wins all of them, then his initial rating will be 2100 according to FIDE rating calculator and FIDE documentation confirms this:
If he scores more than 50%, then Ru = Ra + 20 for each half point scored over 50%

Here Ru is the (new) rating of the unrated player and Ra is the average rating of the rated opponents.
Clearly, this player performed much better than the calculated rating (any performance calculator admits that). And I believe many before me have seen such an issue and such a performance (or maybe slightly weaker than that) from an unrated player is not impossible since many just don't play rated games. I'm wondering what is the rationale behind this? Does it mean that FIDE wants players to play enough many games before they win any title? Or is there any other reason to not use performance for calculating the rating? Were there already some discussions about the initial rating calculation?
I'm wondering what is the rationale behind this?

The rationale is very simple. 5 games is far too few to get a realistic and accurate assessment of a player's strength. That is why a new player's K factor remains at 40 until either they have completed 30 rated games. Additionally it remains at 40 for all players under the age of 18 with ratings under 2300.
Here is the relevant section from the FIDE Rating Regulations effective from 1 July 2017:

8.56 K is the development coefficient.
K = 40 for a player new to the rating list until he has completed events with at least 30 games.
K = 20 as long as a player's rating remains under 2400.
K = 10 once a player's published rating has reached 2400 and remains at that level subsequently, even if the rating drops below 2400.
K = 40 for all players until their 18th birthday, as long as their rating remains under 2300.
If the number of games (n) for a player on any list for a rating period multiplied by K (as defined above) exceeds 700, then K shall be the largest whole number such that K x n does not exceed 700.

A K factor of 40 allows for very rapid changes in rating. Hence the last clause above which seeks to limit the rating change in any one month to prevent see-sawing rating.