I won't say this is a perfect argument, but I think the issue comes down to the fact that forgiving loans is incompatible with the common Puritanical view in America that hard work and delaying personal gratification are virtues. It's not that anyone is being punished but a non-equal distribution of wealth feels unfair, since a lot of people would have made different decisions if they had known that debt would be restructured or forgiven in some drastic way. There are a bunch of people that lived on tight budgets, or worked extra hours instead of taking on debt, or just didn't go to college at all. Those people delayed personal gratification with the expectation that they would be better off later. It is similar for the case where someone's parents paid for their college, their family took on a financial hardship and get no reward compared to families that didn't take on any burden and took out loans. You can imagine how unfair it feels to those people if they saw someone else get $100k in loans forgiven, but they receive nothing. Nobody got punished but someone else gets a leg up and the people that were being "virtuous" get nothing.

And consider this other view: It's not a perfect analogy but imagine that the federal government decides to give $10k to every white man. Of course everyone would scream that was racist and sexist. But nobody is being punished by giving a reward to one race and gender and not the others. There are obviously issues with the analogy, but it might at least show another angle of why some people don't want others to get loan forgiveness when they get nothing.