>>537010512
Playing a champion you already know how to play allows you to learn more about the macro of the game, whether its a simple or complex champion.
It's because the champion has become muscle memory and there's room now to think about other things in the game or smaller stuff (if you actually allow yourself to think about smaller things anyways).
Playing a simple champion in higher elo is arguably harder than playing a complex champion at higher elo because the gap for outplay potential is much smaller in a simple champion when everyone is assumed to be the same skill level regardless of complexity or simplicity of the champion played; That means a higher level of outplaying has to be done that isn't normally done, like maybe cancelling an enemies auto attack by entering a brush after using your combo multiple times, and other things.
None of that means anything though for iron bronze silver gold platinum emerald diamond or who knows where really, but its something to think about. You don't see challenger elo garen only players because there's not enough in the champion to not get destroyed by everyone else regardless of strength of the champion vs another champion.