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.

An informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.