Obviously the reasons to forgive student loans include a wide range of reasons that go from ethical to economic.
First of all, students have no income, so it is unfair, punitive, stratified, and creates dynasties to charge for education at all.
Second is that clearly we do best economically if we have the most educated work force, that allows us to beat the international competition.
If we then make higher education free, it forces government to exert more influence on tuition, which has become extortive.