When home grown professors are inadequately prepared, one gets staff where one can. Or, just maybe, home grown professors think they are worth more than the schools think. They are not hidden, they are in the class rooms every day.
So basically, public workers are exempt. We tried to bring in a few H-1B workers but it was a waste of money. Lots of paperwork. I had a foreign professor from Kenya, he was a sexist pig. Accused me of write Israeli propaganda.
Or maybe many domestic professionals (in engineering and sciences at least) find it more financially beneficial to work in industry rather than academia.