Why are student loans usually guaranteed by the government? ?

The reason why student loans usually guaranteed by the government is: Banks don't have any collateral for student loans.

The recipients of students loan will be most likely still don't have any form of income in their pocket, so the bank sees student loan s something that gives them too big of a risk. The government on the other hand, is less driven by profit compared to the bank, so they could afford to guarantee it.