Because that is an herculean task, given the hardware semantics, the amount of languages that target PTX, the graphical debugging tools that expose every little detail of the cards like debugging on the CPU, and the libraries ecosystem.

Any Cuda-compatible software layer only has two options, be a second class CUDA implementation by being compatible with a subset like AMD ROCm and HIP effforts, or be compatible with everything always playing catchup.

The only way is to use middleware that just like in 3D APIs, abstract the actual compute API being used, as man language bindings are doing nowadays.