"grimy", I thought you were talking about giving people physical exams, and the people hadn't taken baths first or something.
As for getting old exams from upperclassmen, most of them are saving it for the frat brothers/sisters and if you aren't in their frat, they don't want to share. As for why the GDI's wouldn't share, not a clue, maybe they want you to buy them a pizza or something for the old exam?