I think this is just a lack of planning and options for many students. Think about it - it's the end of the year, you've got a pile of exams to study for, and then a week or two later you've got to be out of the dorms. Many students don't have a car to haul things to Goodwill or sell them further afield, and nobody on campus wants to buy their stuff because everyone else is moving out at the same time. If they look into shipping the stuff, they find it's prohibitively expensive. So the only option available in the time they have is to trash it.