Probably the software needs to be optimized for the hw and also the hw may not be general purpose enough even if offered. People demand nvidia because cuda is very optimized for their gpus and many AI software use cuda
You come up with a clever ASIC that is better than their current GPU for your workload… and by the time it comes out they’ve released the next year’s chip that just has like 50% more memory bandwidth or something ridiculous like that, and beats you by pure grunt.
“No replacement for displacement” actually seems to be true in compute.
This is a popular myth. Bitcoin asic's were 'shipping' in 2012/2013.
Some companies definitely played games and mined with the asic's themselves (and then shipped those used asic's)... but in general, it was always a lot more profitable to sell the shovels than it was to mine the gold.