I don’t think it’s quite as simple as you say. This isn’t something like “my childhood was tough, therefore I want everyone’s childhood to be tough.”

Just because someone has paid off their college debts does not mean that the effects of that debt have ended. Obviously the effects of that opportunity cost still exist. That money could have been spent on other things, like a mortgage. So the idea of forgiving all current student loan debt without any consideration of people who are still living with the effects of their previous student loan debt is pretty crude, and I think there’s a strong argument for calling it blatantly unfair.

This can go either way, though. It's just as easy to argue that cancelling student debt is a highly effective form of economic stimulation. There are droves of people who are actively putting off large purchases like houses and cars, and delaying having children, almost entirely because of student loan debt. Removing a huge ongoing monetary obligation from a massive section of people who want to spend is a lot more likely to help the economy than more tax cuts for the already wealthy who are probably going to hoard that wealth away.

Why not just give everyone 100k, people could buy houses cars, etc.. it'd really help those who couldn't even dream of going to college, even with loans. It'd drive up housing and everything else.

That is candidate Andrew Yang's proposal. 120K to every American over 10 years. You should checkout his proposals. There are long form interviews on Youtube. https://www.youtube.com/watch?v=cTsEzmFamZ8