Is there any reason why Fizz isn't disabled in URF?

fizz isnt even too strong, hes just one of the many champions in urf that snowball extremely hard (pretty much anyone with burst and mobility)

Riot wants a permaban list but doesn't actually want to manage it, so they put two problematic champions on it years ago and never touched it again.

That or it's bugs, but if it was the case, I don't see why they don't just add a small reminder each time that these champions are simply too buggy for the game mode. Although even then, I feel like depending on the bugs in question, you can reach a middle ground and just modify their kits slightly in URF to remove said bugs.