They cost quite a bit. Someone has to do all their HR paperwork, show them where the mail room is, support their computer and their phone, keep track of their working hours. Having worked with an intern before, I can tell you for a fact that the constant question is always whether you can find something that the intern is capable of doing that is actually useful to do. Most of the people in my department have a great deal of experience, and so our intern was enthusiastic but needed a lot of help putting together any of his ideas so that they could work. This took paid employee time. So I think you’re overvaluing youthful enthusiasm, especially given the cost of experience that it takes to make the intern’s ideas workable.