AD itemization DOES have 1 thing going for it though (and literally only 1 thing, however it is a pretty darn big thing).
Armor Penetration is incredibly easy to itemize for (far easier than it is to get for an AP mage), and there is no drawback to building the items that have armor penetration because all the stats on each of thee items is useful.

Every single AD champion can build all 3 of these items (and literally every single AD assassin DOES build all 3, and most of the melee ADCs build at least 2 of the above). All of these items are also fairly easy to build (Brutalizer is one of the best earlygame items in the game, and it builds into 2 of the above items, and LW is pretty darn cheap)
Then we consider Magic Pen (and while yes, it is probably too strong, this is largely because void staff is stupidly broken.....however Last Whisper is just as broken)

Most AP champions don't actually stack more than 1 magic penetration item (and if they do, it is because their kit has very strong synergy with the other item....such as Teemo building Liandries + Void), but most of the AP champions don't do so because they cannot make use of the other stats that the item provides (or the passive). A big case of this would be Wits End and to a lesser degree Abyssal Scepter. There are an incredibly few amount of AP champs that build Wits End, and most AP champs don't build an abyssal scepter because the radius for the aura is too short for them to take advantage of it.
This doesn't mean that Magic Penetration isn't a problem, because it most certainly is! However I feel that magic penetration is more easily solved by simply adding a couple more magic resist items (such as some very heavy MR items, the way that AD has heavy armor items like Frozen Heart and Thornmail) and reworking void staff (because lets face it, you literally have to make excuses to NOT build this item on every AP mage/assassin).