It also wouldn't surprise me at all that they do this given the sheer complexity of booting XNU on an Apple SoC. XNU _requires_ that it can talk to a bunch of h

It also wouldn't surprise me at all that they do this given the sheer complexity of booting XNU on an Apple SoC. XNU _requires_ that it can talk to a bunch of hardware early in boot (which, for what its worth, is partially why booting XNU on non-Apple platforms is such a pain in the ass!), and unless you want to run the entire SoC on FPGAs for the _entire_ development process, you need a kernel that can make due with just a single lonely CPU core and nothing else. Linux is designed to run on everything, and XNU is designed to just run on an Apple SoC.