College is very expensive in the US, with the average “in-state” school costing $26K/yr for someone who lives on campus and $56K/yr for a private institution. Since a majority of undergraduates are teenagers with virtually no savings, they take on loans to pay for college. Also unlike traditional loans where you can declare bankruptcy, these loans are very difficult to get forgiven.
There obviously a lot more to it, but that’s the gist in three sentences.