my suspicion is that it is the company culture: the hardware engineers are the real engineers. software is a triviality left for the lesser minds. the consequence is they mess up every product... everything they do needs software.
Essentially it forces AMD to play by NVidias rules, exactly like how they were forced to follow Intel rules. (Ignore for a second that the API / ISA boundary is different.)
But despite that, I also believe AMD would be better off just implementing CUDA.
I can't imagine the logic involved in "this is implemented, let's toss it in the dumpster" for that.