It's not all that hard to make an AP Item for Assassins that won't be abused by mages

etc. all want to be up close and personal to your face while and friends all want to be a medium/far distance from their target(s).

So if you have an item like the following: Ultimate Edgelord Item: 60 AP 10% Movement Speed Unique Passive: 10 Magic Pen Unique Active: For the next 6 seconds (3 seconds for ranged champions), damage dealt to champions within 250 units of yourself does 15% bonus true damage.

Then mages don't really have the opportunity to abuse them. Mages tend to not care about MS all that much while Assassins love it and buying it only for the AP and Magic Pen is a waste when you can get other items that offer a lot more for similar stats. The active is hard to use on ranged champions to begin with because of the range restriction and having the duration be halved for ranged champions makes it all that harder to abuse. This is just a random idea for an item and the one I suggested could be broken/really bad, but that's the general idea. Use the fact that mages like to be away from their targets while Assassins like being close to create these kind of items.

Obviously, there are going to be some mages that could make use out of these items because they are still good even when close to other enemies, but making an item for a class that only has one actual item for them in exchange for a few mages also being able to use it isn't a bad tradeoff. It's just kinda annoying how AD Assassins get all of these toys to have fun with when Riot does updates to Assassins, but AP Assassins are left with nothing new.

End Rant.