Professors don't make a ton of money, in the main, and it's a long haul to find a position and then gain tenure. As for grad students, you get paid either to teach or be a research assistant. It's certainly a pittance, but it's liveable, usually. Contrast this with med school.
I saw a couple grad students transfer out, both times because their professor left, so they likely didn't lose anything. Professors can be pretty mobile, if they get a "name", they are subject to being poached. Usually when a grad student left, it was because they failed something important, like the prelim oral exam. That happened fairly often.
