They're very complex, very stateful devices which also run their own compiled shader code. Not to mention auxiliary DSPs like video decoders (not sure if M1 has it as part of GPU or a separate block), power gating control and many many more.
They may have in order of 100 registers to talk to them and they're horribly proprietary with pretty much no standardisation.
Reverse engineering that is hellish at best - you can see projects like nouveau which barely manages to get nVidia cores up and working without help from the manufacturer. And that's after years and years of development.
The hurdle Noveau is facing is that some things, like reclocking, need firmware loaded onto the card. The firmware is not in non-volatile memory on the hardware, but a file shipped with drivers, the one shipped with proprietary drivers is not redistributable and if you wanted to make your own, it needs to be signed by Nvidia anyway.
That's pretty much game over for Noveau, and it is not due to difficulties in figuring out registers and NV ISA.
They may have in order of 100 registers to talk to them and they're horribly proprietary with pretty much no standardisation.
Reverse engineering that is hellish at best - you can see projects like nouveau which barely manages to get nVidia cores up and working without help from the manufacturer. And that's after years and years of development.