because by playing passive you rely on
a) Getting an advantage in CS therefore having more items
b) outscaling your opponent
a) just stops working after a certain ELO where everyone is capable of CSing properly + when everyone's Cs is equal, the one with kills will be stronger
b) you are relying on playing a champion that scales really well AND your opponent not abusing your weak points.
Also just the general fact that by playing passive you essentially just wait for a late game teamfight to be won, and lategame teamfights are pretty much a 50/50 chance, not to mention if your teammate gets caught out the game is over anyway.