not everyone started playing in s1 to know all roles "even decently".
Also, the higher in elo you get, the more you would find people are actually "specialized" on their roles.
Even in LCS, you don't see mid laners playing support every 5th game. You don't see ADCs switching to top lane every 3rd game.
The higher in elo you get the harder is to keep a large champion pool for all roles.
I'm plat elo. I got ~3-5 mid laners I can play at that elo range, I got 2-3 top laners I can play at that elo level, I got 1 support (which I refuse to play when auto filled) around that elo range and 1 ADC.
If I would have to even go on my main role (mid) and play Azir/Oriana/Cassio/Ryze/Annie (and everything else I don't know how to play with) in ranked, our team has 80% chance to lose. If the enemy team bans Janna or I'm pissed because outfill, our chance of losing are ~90%.
If the enemy bot lane counters MF, I'm useless for my team even playing somewhat decently.
If I were to go higher to Dia. My Janna would fade away. My MF would become weaker and I would be left with ~2-3 mid laners and 2 top laners.
You need to realize this game has a lot of information.
And you knowing that information takes time to acquire. That's why people got mains. With those specific champions they are aware of their damage output on different stages on the game, they are aware of different builds and playstyles they can use, they are aware how they can avoid damage, they are aware of what they can actually do and what they can't do. And all that for a single champion is like doing all combinations possible of 5 out of 120+ character roster.
Its A LOT of information. And there's also wave manipulation, trading, zoning, sieging, baiting, vision control. Even here I'm plat and not doing some of these things even on my mains. Coz I don't know every match-up yet...
Its literally impossible to ask someone that is playing at his peak on one specialized role to instantly be able to play, also as his peak, another role.
If I'm facing a non mid/top main on mid while using my main... the usual result is a 1 sided game (unless enemy jungler camps the shit out of me) even if the other guy is 5 divisions above me (and the difference between plat 3 to dia 3 is huge).