Therefore they can continue to extract more from people than before. When you default, the amount owed increases significantly (doubles?).

This means more people are stuck with high payments and are required to work to continue to survive, less people getting retirement or other benefits, a larger workforce that is essentially captive to their debt and can be more easily manipulated because they are worried about making ends meet more than other things.

I see plenty of benefits for capitalists in a wide variety of industries if more people are beholden to more student debt.

There are entire industries that profit from people being imprisoned. The more people incarcerated in a location, the more that location can charge.

All other things aside, there is very much a profit motive.