Because mircosoft let you run their OS on anything at all and apple uses a more "closed" system where they control both the software and hardware. They have been using both of these ideals since the beggining and its probably why windows is where it is today. I dont think apple is just going to let sombody comprimise their ecosystem after they have been controlling it for like 20 years.
There is also some sort of law against it that apple set up making it illegal. Im not sure how well they enforce it but they sure as hell wouldnt tollerate some major chip maker doing it.