With AD itemization, there are different items for different classes and types. ADC rework is trying to broaden the diversity, but other build paths can be viable (On-hit, caster/adc, blue build, etc). There's also AD items for fighters that help give tankiness, and AD items for bruisers, and AD assassin items... With mages, you've got a single core of items that all do relatively the same thing. There aren't items that are better for long range, and then better for divers/assassins, they all build generally the same thing.
Part of the problem is that there is really very few optimal AD items, and part of the problem is purely just that there's far less options, and even the options that we do have feel very similar (playstyle wise, void staff feels like deathcap, which feels like morello's and liandry's, etc. None of them change how you play.) If you look back over the years, there have been significantly more AD items added than AP. Aside from the hextech belts, there hasn't been any legitimately new AP items in ages.
Could we get some better item diversity for APs?