A really high ban rate is a sign of serious champion health problems

Any time that a champion has an insanely high ban rate (certainly anything over 40-50%), it's a clear sign that there's something very wrong about that champion.

No one wants to play against them or to play the game with them in it. Either that's because they are vastly overpowered, they are absolutely no fun to play against, or they have too many extremely binary matchups (e.g. hard-counter many champions, get wrecked by others) - or often all three of those things at once.

People like to talk about win rates and the importance of balance, but really, that's only part of it. The problem with being overpowered (and performing too well) is fairly obvious. When people absolutely don't want to play against a champion because they are simply not fun to play against, it is probably more likely to drive players away from the game entirely if champions like that are reasonably popular. When a champion has extremely binary hard-counter matchups, then it can warp the entire shape of the game and which champions can be picked ("I want to play X, but they haven't picked yet and might pick Y, so now I have to play Z just in case.")

Any of the above three problems are major issues with a champion that need to be urgently addressed. Why aren't these champions Riot's top priority? Instead, they often get left alone for patch after patch, with no or only minor changes.

It's not fair for the people who want to play a fun game without it being warped by the presence of those champions, and it's not fair on the people who actually want to play those champions, since they are always banned and thus can never be played.