Rito keeps lowering effectiveness of AP items, and they wonder they get AP Assassins building tank?
Over course of years, almost all AP items got either their cost efficiency, or their slot efficiency severely reduced. If you gut AP item pool, at certain point buying tank items becomes more efficient. That simple.
First indication that something is wrong was midlane
domination - the item was strictly superior to anything a mage could rush, even though they couldn't utilize it at it's full potential.
Now, many AP champions got buffs to compensate, right, but this doesn't change the fact that if you can avoid building AP items, you most likely should. If your kit is strong enough to function with weak AP items, it will most likely be even stronger with powerful tank and bruiser items.