Problem is, inference costs do not dominate training costs. Models have a very limited lifespan, they are constantly retrained or obsoleted by new generations, so training is always going on.
Training is not just matrix multiplications, given hundreds of experiments in model architecture, its not even obvious what operations will dominate future training. So a more general purpose GPU is just a way safer bet.
Also, LLM talent is in extreme short supply, and you don't want to piss them off by telling them they have to spend their time debugging some crappy FPGA because you wanted to save some hardware bucks.