because no matter what team they're on they're usually toxic asf
If they're on your team they spam ping your jungler and shit talk everyone for feeding or not camping their or w/e. If they're on the enemy team they do the obnoxious thing and spam mastery emotes, laugh, etc, and maybe say in /all chat "you're trash". You're not allowed to have fun if riven is in your game, and god bless every good well mannered riven out there that can break the stereotype of their champion.
I mean
is very similar but at least a dumb or bad vayne can still get a penta kill if they farm. You peel for vayne late game and you're guaranteed value unless the guy just suicides. A bad riven is pretty much completely useless to your team.