Well... how else do you pay back the loan you take to study in USA? This is a problem for a huge portion of students, immigrant or not.

You can argue that students can go back to their own country and repay the loan? That is practically impossible for almost every student coming from India and China.

What happens to a foreign student who leaves the US and just doesn't repay their loan? It seems to me that nothing would happen at all?

You can't get a US student loan without a citizen or permanent resident cosigner.

Banks cooperate internationally, you're on the hook both in your home country(or whatever country the co-signer is from)and the country the college is in. it's impossible to just skip out on loans nowadays(unless the trusted co-signer disappears with you ofc)

Foreign students get loans from their home country not the US. No US institution will lend to a foreign student without a domestic co-signer, for precisely this reason.