"Why am I seeing the same champions in every game?"

It's called the meta. There will ALWAYS be champions that are stronger or at least played more often than others.

This is the same for EVERY FUCKING GAME where there's options.

In CoD, there's always a gun that's used more often, like the M16 in Modern Warfare or the Intervention in MW2.

In Smash, it's never long before a stable tier list comes out, with most players sticking to Fox in Melee and Shiek/Zero-Suit Samus in Smash 4.

In Overwatch, there's almost never a game without a D. Va.

In Starcraft, there's always a most-used strategy, like Zerg Rushing in the early days.

This is unavoidable issue, because it's NUMERICALLY IMPOSSIBLE to perfectly balance every character. Some of them are also just inherently more fun to play, or at least more fulfilling. That's just the way games are.