You're confusing device-specific APIs with OS-specific APIs. There's a big difference. DX11, DX12, Metal, etc. are themselves all abstraction layers over what a

You're confusing device-specific APIs with OS-specific APIs. There's a big difference. DX11, DX12, Metal, etc. are themselves all abstraction layers over what are in some cases very different hardware designs. (This will eventually become less so with Apple moving to in-house GPUs, but for now it's still true--Metal targets NVIDIA and AMD GPUs as well as Apple's M1/M2s.) Maybe it'd be interesting to contemplate a world in which red team, green team, and blue team all have their own proprietary APIs and there are no cross-vendor abstractions, but that's not what we're discussing here.