Not to mention that the same amount of gold needed to buy 1 ad can also buy 1.5 ap.
AP has always scaled better than AD; this is to make up for the lack of a critical chance multiplier and auto attack damage increase.
It is also why the only viable AD assassins are those with absurd scalings in their kit such as Zed, Talon, Riven, and Yasuo. While there are a lot more AP assassins or mages with nukes, ie. Fizz, LeBlanc, Akali, Katarina, Kassadin, Nidalee. It is much easier to get high base AP for ability ratios than high base AD.
It is just the way it is. As well, MR items comes with both AP, AD or Health, while armor mostly only comes with health. It is the price one pays for versatility, and a chance of survival in face of a full AP combo.