Why do people think damage is everything?

Honestly, It is so frustrating when you have a champion who's meant to be tank but a teammate decides to go damage on them. Why is the pantheon top going full ad like wtf he falls off late and you constantly tell your teammates that but Noo "Trust me it's op" Then you lose the team fight and he wonders why we lost. Or a yi or yasuo top like at least go half tank, Jesus Christ this is such a piss off. I can't take it anymore I swear some people are so boosted.