https://www.anandtech.com/show/12552/amd-announces-real-time...
I love open standards as much as the next guy but for something like this where the utility is still an open question, I'd rather than some just makes an implementation for just their technology. The alternative is that your open standard is full of half baked experiments and trimming a standard is a huge pain.
If DirectX were to be used on Linux and macOS, you'd have some real competition, but that is not the case.
And most new phone chipsets. And a lot of old phone chipsets. And a lot of ARM-based single board computers. And a lot of non-Xbox consoles like the Switch have hardware support for DirectX...
DirectX is in a lot of places you would not expect.