The trick is, AD scales with multiple stats, making it easier to design items for specific classes.
AP scales with... Well nothing! Some items like Ludens or Protobelt have AP scaling passives but that's about it.
You can make a crit item for marksmen since that's what they scale with
You can make a lethality item for AD assassins since that's what they scale with
You can make an attack speed item for attack speed bruisers or on hit effect champions since that's what they scale with
But for AP it can be used by anyone who builds AP.
There was an attempt in MYMU to make mana for mages and mana regen for supports (wich kinda worked, but mostly because the 4 support item that have AP on it are just too weak for a midlane mage). But appart from that, AP items are pretty much scattered on anyone who scales of AP.
You can see ROA on Tanks like Maokai or Nautilus, or on mages like Anivia, or even assassins like Kassadin.
You can see luden's on burst mages like Lux or Syndra, or on sustained damage dealers like Cassiopeia, but it's also very good on some poke supports like Karma or Morgana.
Lich bane is good for Assassins like Kassadin or Fizz, but also on champion that scale of on hit effects and AP like Kayle.
Meanwhile if you take an item like essence reaver or rapid fire cannon, pretty much no one but marksmen build that.
If you take an item like trinity force, it's pretty much only built on bruisers and Corki/Ezreal
TL:DR: AP doesn't go hand in hand with other stats like AD does, so it's almost impossible to make item for a specific class of AP champions.