Is it? A competitor can enter the market and undercut by producing a cheaper and otherwise undifferentiated commodity-type product. Nvidia's focus is adding moats that prevent competing on pure specs such as CUDA, design, and so on.
Irrespectively though I don't see how a competitor can replicate the context and tacit knowledge associated building something like CUDA for close to 20 years without putting in a similar amount of time.
Nvidia also made sure that CUDA runs on gaming GPUs and supports Windows. This is why its tools are so good. You don't need to buy a datacenter GPU, no need to mess with Linux. Just buy any gaming GPU, install CUDA SDK and you're good to go.
AMD wasn't like that. Their alternative - ROCM didn't even work on gaming GPUs. Their datacenter GPUs didn't even support Windows. Basically the opposite approach of NVIDIA. Now AMD is rushing to add Windows and consumer GPU support to ROCM, but it's a bit too little too late.