You Ever Been In A Game Where Your ADC Does No Damage In Fights
And then you get out of the game and notice that they built no crit chance? Why does everyone want that to be every game? Crits are really the only way an ADC stands a chance. Otherwise they auto for a while until the enemy decides they're done and run away or mistimes it and dies. Crits add a factor of unpredictability. And it goes both way. An ADC might be counting on a good crit and never get it, or the enemy might be hoping it doesn't happen and it does.
Although I do like the idea of every ADC having different on hit effects replace crits. Even though that would be more about counterpicking and less about individual skill, so rito won't go for it.