alt Hacker News1) Intel and AMD help to implement support in Linux before their chips even ship. Actually a sanitized version of the Intel graphics ISA bspec is actually available to the OSS community too.
Apple on the other hand provides no support. The one nice thing they did do is allow their bootloader to boot non-apple signed OSes. They do not do this on iPhones, iPads, Apple TVs, Watches, or homepods btw.
2) The GPU ISA changes drastically and often. Its not entirely uncommon for the entire instruction set to change entirely within one generation. Every change to the ISA would require an entire round of new reverse engineering (I suspect, ive never reversed).
I do wonder why Apple chooses not to lock down the Mac to just Mac OS like all their other hardware? I'm sure the sales from people who intend to run something other than MacOS look like a floating-point error on the scales Apple operates.