IMO the reason Apple doesn't provide this level of hardware documentation is because modern Macs don't have comparable expansion capabilities. The kind that expose system buses on connectors that users are supposed to plug cards into, and third-party developers to interact directly with hardware to make those cards work. On a modern Mac, you've got USB and Thunderbolt that you can interact with from a userspace program.
Though I'm not denying that some of the newer macOS APIs are very poorly documented. As in, you know you've stumbled upon the cool shit when you end up on one of those old pages with a blue gradient in the header that says "Apple documentation archive".
Though I'm not denying that some of the newer macOS APIs are very poorly documented. As in, you know you've stumbled upon the cool shit when you end up on one of those old pages with a blue gradient in the header that says "Apple documentation archive".