People aren’t comparing PPP & student loan forgiveness because they’re 100% analogous.
It’s because in both, taxpayers bear the cost of providing an economic support to some, not all. The two programs do share that feature, even if you argue PPP was more worthwhile/necessary.
This should go without saying, but PPP loans aren't the same as college debt. We faced a natural disaster (pandemic) compounded by government shut-down orders that blocked people from working. PPP is better viewed as a form of disaster relief.