Tell me how you solve the problem of Ziggs and Leblanc.
Leblanc gets the same value out of AP as Ziggs (theoretically speaking, not literally)
But they have completely different play styles.
Do you make an AP item that has when a champion is within 200 range, they take increased damage by 20% for akali, sylas, leblanc, etc
and an Ap item that has when a champion is outside of 800 range, they take increased damage by 15% for Xerath, Lux, Ziggs?
Then you are left with Ryze, Cass, Viktor, Azir, TF, etc. Whom are in the golden range of both sets where they want It to be between 201 range and 799 range. Every AP champion wants that. (Other than Sylas, Akali, and Ekko) But they also benefit from within 200 range, and outside of 800 range. But that is in situational play.
That is just the range difference.
Then when you get down to the nitty gritty.
Do you emphasis continuous spell cast like Ryze, Azir, Cass, Kass, Karthus with an Item like for each spell that damages the same enemy champion increase the damage output by 4% per spell up to 40% increased damage?
Or do you make an item that does a flat 25% increased damage on the spell after clicking R for Syndra, LB, Akali, Sylas?
I am only using increased damage as a consistency.
AP Itemization is really bad not because the items themselves are bad, but because how varied the AP champions are.
Where as in the AD portion of the game, the game items and champions are defined by Ranged/Melee skill sets being locked by the skill set.
Mages don't have locked skill sets like AD champions do. Few AD champions have ranged attacks like Viktor, and few Mages have Dashes like Riven. (Assassins/Divers don't count as mages)