https://www.chronicle.com/article/Graduate-School-in-the/448...
One thing that is often overlooked is that the US does have a very restrictive immigration system for people who don't have relatives here and can't leverage family reunification. In this case, getting advanced STEM degrees provides a path towards residency, work rights, and perhaps eventually a green card or citizenship. So in the US, these folks wouldn't be looking at a choice between getting a PhD and becoming a florist, because it would be illegal to come here and work as a florist. But there's a "shortage" of people getting PhDs in STEM fields, so we allow people to come here under conditions of limited personal freedom provided they agree to work these positions.
This power on the part of universities (the power to bestow residency and limited work right), I think, is one of the reasons STEM PhD programs haven't collapsed or faced the reckoning that would force them to change.