Games are so short because no one wants to invest into the late game

Items, essentially.

Tanks got it easy because their items cost diddly-squat. They'll always shit on ADC's early game.

Quite a few ADC's have lately tended to prefer going straight AD instead of crit, because crit relies on RNG, and there are chances you might just get shit on when you use you have only 40% crit and manage to not get crits seven times in a row. Full Crit will usually outweigh Full AD, but AD is something that crit isn't --- RELIABLE.

For mages, there's an easy way you can get a whole load of AP. , obviously. That gives you tons of mana, some decent CDR, some health, some magic pen, and of course, lots of AP. Do people do this? Goodness, no! People just start , because it's all you need to completely molest an ADC in the early game.

There are other reasons, but items is the point I wanted to make. This is why crit ADC's are "Feast or Famine." They're wasting their money if they don't get into the late game.