Why can't items just be limited to certain champs?

I seriously don't understand why certain items can't just be limited to a certain champion pool. It would fix so many issues with certain champions building certain things that make those things "broken". It's all throughout league's history with adc abusing assassin items, mages abusing support items, etc. Why can't you just Riot just limite items to certain sub classes like "Only tanks/bruisers can build X item". This wouldn't have to be for every item, just certain items that are easily abused if thrown in the wrong hands.