Ability power champions have a vastly bigger range of items to choose from than attack damage champions. For example noticeable ability items that are unique and unique only to AP because AD doesn't have that (Zhonyas, Rylaes, DFG,) AD champs have none of that cool stuff. They are stuck with the usual BT, BC, TF, LW, etc.
In the end: give AD champions more items that benefit with AD
SKT T1 Cashe4 votesto be fair, AD also affects your auto-attacks, which AP characters don't really have the benefit of sense they are busy building AP.
We have a rather good collection of Life-Steal, Attack Speed, Attack Damage, Armor Reduction, Crit-chance, ect...
Even the 
gives massive HP, AD, and the power to slow down enemies
So I say AD characters have it good.
AP characters, on the other hand, require them to be more focused on their skills as those skills require cool-downs and mana. AP characters can also be rather squishy (not counting the tanky AP characters in this) so they have to be careful how they handle their lane.
So in short, I think everything is fine as they are.
Lord Dusteon1 votesMercurial Scimitar, Frozen Mallet, and Infinity Edge. There, you just got the closest AD equivalent to those AP items.
SexyTaylorSwift1 votesPlease stop trolling.
SmokingPuffin1 votes
AP champions would kill for an AP version of any of these. Very impactful unique effects.
