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.
But the issue remains that the actual support and debugging tools remain so atrocious that it doesn't help to combat the CUDA monopoly. They've further burned a lot of trust by never really delivering on their promises to do better unless you're a customer large enough to get personalized attention from their engineers.
This ends up being a double whammy because not only are you pushing away smaller businesses, you're also pushing away single developers that go on to influence purchasing/development decisions.