There is another reason - if the cheap phones necessarily go up in price, but Apple eats the cost increase since they can afford it, then the cheap phones suddenly will cost about as much as an iPhone. That is a big ick.

A big selling point for iPhones is that they are much more expensive than the Other phones, so it is absolutely necessary for Apple to make sure it's phones keep a 50% price luxury bonus, or many will stop buying them.

TL;DR - ultra-expensive iPhones are a feature, not a bug, like ultra-expensive watches