A good number of comments here mention dealing with the GPU is going to be a major hurdle. What makes porting GPU drivers significantly more challenging than everything else?
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.
That's pretty much game over for Noveau, and it is not due to difficulties in figuring out registers and NV ISA.
But that's a bit besides the point :)
I can understand preventing FW patching, maybe. But redistributing the manufacturer's genuine FW, why not?
Multiple reasons:
1) GPU manufacturers are notorious for not publishing documentation out of IP/patent concerns. Worst offender is NVIDIA here.
2) For embedded GPUs there isn't much interest in open source drivers... the big customers (think Samsung and the likes) have direct support from the chip design vendor and get drop-in drivers as part of the board support package (BSP, basically a conglomerate of bootloader, kernel+modules+initrd, firmware blobs for components such as wifi) so they don't need OSS drivers
3) The mobile GPU space is... splintered. With desktops you got the three major players AMD/ATI, NVIDIA and Intel's built-in Iris, in the GPU space there are more.
I think easily Apple takes the cake from nVidia - they don't even provide drivers for anything but their platforms (that is for their proprietary GPU core). The GPU core that's actually in the M1.
Making any modern GPU work is a lot of work because of how complicated they are. That's even with the full documentation.
In the Apple M1 case, the GPU will have to be reverse engineered to understand how it works, then a driver will need to be written for Linux that supports it.