Metal is the answer. Everything else is just implementation detail as GP said.
Apple doesn’t provide developer support to other OSes. The only OS they do anything for* is macOS. So to them there’s no point.
All they’d get is people relying on implementation details they shouldn’t, other companies stealing what they consider their trade secrets, or more surface area for patent trolls to scan.
* Someone on the Asahi team, I think Hector Martin, has commented before the Apple is doing things that clearly seem designed to allow others to make and securely boot other OSes on their Apple Silicon hardware. They clearly could be clamping it down far more but are choosing not to. However that’s exactly as far as the support appears to go.
You can say this as long as you want, Nvidia makes money hand-over-fist supporting CUDA alongside OpenCL and DirectX. It's all just business to them - they don't have to play the same game as Apple because they're just not quite so petty with the ecosystem politics.
Look at MacOS, for example. Plenty of legacy software never was supported in Metal, it's "implementation detail" never manifested. It wasn't even really used in AI either until Apple upstreamed their own MPS hacks into Pytorch and people got BERT et. al. working, and even that was a pint-sized party trick that you could do on a Raspberry Pi. Apple themselves aren't even using their own servers for serious inference either, because you can't. It's gotta be offloaded to a lower-latency platform.
It's not just that Metal as a platform has failed it's users, although it's certainly contributed to developers giving up on Mac hardware for serious compute. Apple's GPU design is stuck in iPhone mode and they refuse to change their approach with Apple Silicon desktop hardware. It was Apple's big bet on NPUs that hamstrung them, not an implementation detail, and if you don't believe me then wait and see. Xserve didn't tear down the 1U market, Asahi didn't upend Linux HPC, and Metal isn't going to upend AI compute any more than DirectX will. This is the same "Apple will get 'em next year" quote we always hear when they fuck up, and they never actually seem to swallow their pride and take notes.
What is the main reason?
I can't guess what is the main reason. There might not even be a main reason, as many groups of people at Apple and its shareholders decided this over the years.
(Also see my speculations below in this thread).
So not in any order of importance to Apple:
1) Create the same moat as NVIDIA has with CUDA.
2) Ability to re-define the microcode instruction set of all the dozens of different Apple Silicon chips now and in the future without having to worry about backwards compatibility. Each Apple Silicon chip simply recompiles code at runtime (similar to my adaptive compiler).
3) Zero hardware documentation needed, much cheaper PR and faster time to market, also making it harder to reverse engineer or repair.
4) Security. Security by obscurity
5) Keeping the walled garden up longer.
6) Frustrating reverse engineering of Apple software. You must realize Apple competes with their own third party developers. Apple can optimize code on the GPU and ANE, third party developers can not and are forbidden too by Apple.
7) Frustrating reverse engineering of Apple hardware.
8) It won't make Apple more sales if 3rd party developers can write faster and more energy efficient GPU and NPU software.
9) Legal and patent infringements considerations
10) Future compiler improvements
11 ) Trade secrets
