AP and AD itemization are not the same, because the champions who utilize them, and their roles within the game, are completely different.
ADCs are, in general, encouraged/forced to build primarily offensive, with maybe one or two defensive items. This is because if they can build damage they are a massive source of damage throughout a teamfight - the raw sustained damage of a High-AD, Crit-hitting carry with Lifesteal to sustain them makes them a necessary part of every team comp. They take down towers the fastest, they deal the most reliable damage, and so on. Part of the balance to this is to force them into a "risk vs reward" build path - the offensive items they want won't give them the defenses they need.
Basically if ADs had easy access to AD/Armor, they'd just stack that item and call it a day. In early/mid game, the biggest threat to the bot lane AD is the enemy bot lane AD. So you rush that item, and then you get into a race against the enemy where you're both just stacking as much of it as you can because there's no reason not to - you get both stats you need to kill the other.
APs have a little bit of this but not in a way that would be abused like the ADs. For one, AP paths tend to be less rigid. With few exceptions, ADs build very predictable items. PD/IE/LW/BT is pretty much standard for your average ADC. Sure, an AP can rush Abyssal, but Abyssal isn't always the best item for APs. To get the MR reduction you have to get close to your target, so that part is ineffective if you're a long range mage. And if you're a High-AP Ratio mage that wants to rush a Rabadons, you may not rush Abyssal. Same for mages who need to rush the Hourglass for their ultimates or to survive a bursty enemy.
Even in terms of boots. As a Mage V Mage. Build Sorcs, and be squishier, but deadly? Or build Mercs, and be beefier? And what if the enemy has CC, but you want burst? Where as ADs almost always just go Berzerkers.
Also, APs tend to be more focused around burst - cast a few spells, throw all your shit on CD, and then hope you live long enough for your next spell rotation. So having defenses during your "down time" is more crucial for mages. ADs, on the other hand, never have downtime because they can always AA.