Why are automobile manufacturers trying to kill the stereo aftermarket?

By that, I mean that it is becoming increasingly difficult to fit an aftermarket stereo into your new car. Manufacturers are making nonstandard sized stereos that are physically difficult to remove and hard to fit a replacement to. They are tying unrelated in-car equipment to them, making them a requirement in the car (which brings the additional question of what if the radio dies). I'm sure there are other things, too, but these are the prime problems.

But why are the manufacturers doing this? Are they actively trying to kill the aftermarket? Or do they think that people really need or want their stereos to be oval or rhomboid? Do they think we need our gas mileage to be viewable on the radio?

This is really pissing me off. I have the feeling that in a few years, it will be basically impossible to replace the factory radio.