considering how opening the Mac platform up to third-party HW in the 90s was one of the things that almost drove Apple to bankruptcy, you can see why they're not keen on making that mistake again. they're a HW company and can't undermine that.
what's funny is that they're more normal - historically speaking - than Microsoft. the *usual* practice was for the HW vendor to also provide the OS, and often even the software.
Apple is just doing what everybody was doing prior to the era of x86 machines.