There's that and the fact that a lot of people who attain graduate degrees are immigrants who do so for the sake of immigration.

The whole system essentially self selects for cheap labor and exploitation.

If the feds put a high salary requirement on it like the E or O series visas, perhaps the system might change.

The scientific minds of India, China, and Russia don't come to the US and slave away in the lab purely out of passion for advancing science, they do so because it's a path towards the green card. The PIs and laboratory heads all know damn well how the system works, they are no better than those bosses of H1B sweatshops, except perhaps they do their exploitation from ivy filled ivory towers rather than in Patagonia vests.