How can a foreign student in the US get a student loan without a co-applicant (US citizern/ permant resident)?

Answer 1

Collateral. If you don't have it, no one will lend money to you. If a foreign student in the US borrows money, gets educated, then returns to his homeland, the creditors would not be able to reach him to collect the money owed. He could walk away from his obligation and not suffer any consequences.

Answer 2

You wait till you become a permanent resident or citizen.