Intel sells parts to other people, and those buyers expect that kind of documentation. And when it comes to GPUs they’re in 3rd place for high end GPUs and are incentivized to give people as much info as possible to optimize for their hardware.
Apple doesn’t sell their computer hardware to any other OEMs, or hsve a 3rd party making their OS, so they don’t need to produce documentation for anyone.
I agree it would be nice if it was documented. But absent a law forcing them I don’t see it happening.