It's not that hard--you must provide a way to use CUDA on your hardware. Either support it directly, transcompile it, emulate it, provide shims, anything. After that, you can provide your own APIs that take advantage of every extra molecule of performance.
And neither AMD nor Intel have thrown down the money to do it. That's all it is. Money. You have an army of folks in the space who would love to use anything other than Nvidia who would do all the work if you just threw them money.