MR for AD vs Armor for AP
AD champions have a great many ways to survive against AP and damage in general:
: 
AP champions have:
: 
(arguably Zhonya's Hourglass is the best item in the game, but in comparison to all the other items above, which passively increase your chances of living through a fight, one good active doesn't compare).
And there are plenty of AP items with HP: 
but that only makes 
and 
do more damage. AD champions have plenty of options for dealing with magic damage while also building offense, while AP champions just have to be sandbags until the enemy goes all in.
TL;DR: Mages need more options!
