You’re being unperceptive and going drotesque generalizations that aren’t to the point. It is quite clear that some games have more toxic communities and some less and it is directly linked to the design of the game.
Your thinking is akin to thinking that the crime rate is something the society should handle on its own rather than direct responsibility of the government.
Balance matters. Check out stats at raiderio with tanks and healers. While for tanks there are several popular alternatives at high-level keys, for healers there’s only Shaman that rules them all.
While community might be problematic, the community preferences often driven by real disbalance.