Yeah but then you have to do the research that you have all the correct drivers and so forth. Also with time things such as harddrives deteriorate. And also by your same logic. If you are able to get computers for free then you would also be able to get the keyboards, mouses and displays for free right?
I think you are missing the point. Doing that for one machine for yourself: easy, cheap, maybe even fun. Doing that for 5-10 unknown machines for other people to use in a class... Each may be different, you have to setup each one, they may fail or misbehave at any moment, even during class... Not my idea of easy, cheap or fun. Talking from experience in exactly that kind of situations, working with donated hardware in a school setting.
