The fact that ARM has not made their Mali drivers open source is a joke. There's no excuse, same for Nvidia.
Their driver are not merely to "communicate" with the hardware. The driver include a lot of hotfix and optimization, sometimes specific to one game/application. The driver give them a competitive advantage over their competitor. They will probably never released it for free.
Interestingly, amdgpu/Mesa performance seems to be higher for typical game rendering than AMD's proprietary stack. I suspect at some point in the future AMD will abandon their 'pro' proprietary stack entirely after realizing they should stick to being a hardware business.
Now, with NVidia diving more and more into the ML / GPGPU market (which is less windows-focused), they might have more pressure to actually open-source their driver (or at least, finally make them actually integrated properly with Linux), at least for their server focused hardware. But I wouldn't bet on it any time soon.
Let's be honest, they have a dominant position on the market. And their driver, as awful as they are, work. I think a lot of things would have to change for them to have the incentive to open-source them. My only hope at this point is that, with the growing GPGPU market, they might finally start to see Linux not as a second-class citizen.
John Bridgman of AMD confirmed it's their plan like 5+ years ago. The only reason they ever maintained proprietary stack on Linux is workstation users who need certified OpenGL implementation for various proprietary software.
By now the only proprietary parts of "pro" stack are OpenGL implementation and shader compiler for Vulkan / OpenCL. Everything else in "pro" package is open source already. There is still legacy OpenCL implementation, but I guess ROCm will be used for everything soon.
They are also entirely useless without the userland software that is doing all the actual work. The kernel drivers are just glorified task and memory managers.
I thought that the Lima and Panfrost projects included an open-source userland component too.
And of course just generally vendor kernel code tends to have superfluous middle layers or abuse kernel interfaces in unintended ways.
Given that the most frequently cited reason that these drivers should be open source is improved user experience (less breakage when the kernel changes, easier install), what would be the advantage of an out of tree open source driver?
However, my experience in the hardware industry leads me to believe it's something about an 'intellectual property' fetish and pearl-clutching when confronted with the idea of 'giving away code for free'.
e.g. when AMD open sourced AMDVLK, they had to replace the proprietary compiler it was depending on with LLVM. That was an easy one. Many projects are much more deeply intertwined with proprietary crap.