Graduates do pay interest, even if the original loan is never paid back. The amount they pay out of their incomes is almost all interest.
Student loans aren't like other loans such as a mortgage. They are more like a graduate tax.
The reason so little is being paid back at the the moment is because we have had wage stagnation, particularly in the public sector, which is where many graduates work. If there is wage inflation in the future, more will be paid back, but inflation causes problems in other areas.