Why are avoidable openings played instead of systems? It's been a while since I've played OTB chess, but I've gotten back into playing it online. Before my break (I was rated ~1400 USCF), I was a heavy advocate of the English Opening:
rnbqkbnr/pppppppp/8/8/2P5/8/PP1PPPPP/RNBQKBNR w kqKQ c3 0 1

[Event &quot;Local Tournament&quot;]
[Site &quot;?&quot;]
[Date &quot;????.??.??&quot;]
[Round &quot;Opponent&quot;]
[White &quot;BaroclinicPlusPlus&quot;]
[Black &quot;?&quot;]
[Result &quot;*&quot;]

1. c4 

with the idea that instead of seeing any opening prep that I make not being thrown out the window, I could force my opponent into lines that I felt comfortable playing. For example, if I was preparing the Scotch Game, and was faced with an opponent who played the Sicilian
rnbqkbnr/pp1ppppp/8/2p5/4P3/8/PPPP1PPP/RNBQKBNR w KQkq c6 0 2
[Event &quot;?&quot;]
[Site &quot;?&quot;]
[Date &quot;????.??.??&quot;]
[Round &quot;?&quot;]
[White &quot;BaroclinicPlusPlus&quot;]
[Black &quot;Well Prepared Opponent&quot;]
[Result &quot;*&quot;]

1. e4 c5 *

I'd be in murky waters. So to prepare for the Scotch Game, I'd have to learn the Sicilian Defense, Caro-Kann defense, French Defense, and any other defense I could see.
My question is this: If you do not know your opponent ahead of time, what advantage is there in preparing openings like 1. e4 when your opponent can nullify any preparation?
That is, most opening prep for white relies on Black playing 1. ... e5, which is not only avoidable, but also lines that I am not usually prepared. The same can be said about 1. d4 and the Queen's gambit (ignoring the increasing popularity of the London System). I've tried moving beyond the English, just to be left with either needing to learn a ton of theory and opening traps or face a better-prepared opponent.
*Speaking from experience, where I have a Black defense for 1. e4, 1. d4, and 1. c4.
When chess players reach a certain level, they will know how to play against non mainstream openings(often also they can figure many things over the board as well). Therefore, if you play anything other than e4,d4,Nf3, or c4, the black player will be fine as the other moves are objectively not as good and white will find it hard to get any edge. So basically the advantage of playing 1e4/1d4/1Nf3/ or even 1c4, is that they are objectively good and offer white good chances to fight for an advantage. While if you play something like 1g4, you will not be able to fight for an advantage at high levels(in fact, you will be struggling to not lose).
That is, despite your opponent having knowledge about how to play against 1e4 or 1d4, it's still a good idea to play 1e4 and 1d4 because those first moves are that good, and you also have knowledge of those openings. If instead you choose to play things like 1b3, 1f4, strong chess players will equalize easilly against them.
At lower levels, it very much would be advantageous to play something people don't know much about. So at the lower levels, you can have a repertoire of unsound gambits and tricks, and you'll be fine. But of course, if you face stronger oppositions, you'll have trouble.