Actually it has nothing to do with the age of adulthood. It has to do with out of control liability law, some specific federal legislation relating to students' "privacy rights", federal (and state) funding which enables many students to attend schools without their parents' consent (e.g. if parents are deemed to poor to afford to contribute significantly to college costs, the taxpayers are forced to pick up the whole tab, while the horrified parents watch their kid go off and major in "Queer Studies" or "Creative Writing"). This results in most colleges treating students and potential students as the market they must sell to, and accordingly to both curriculum and rules being designed to appeal to 17-18 year olds.
I hear your points, but I think changing the legal age of adulthood would cause the law to treat 18, 19, and 20-year-olds as minors and there would be more control. If they can't drink until then, then they might not be ready for voting, serving in the military, or signing contracts.
I think these two things are critical. In the case I described, I'm sure these were the concerns of the college. If "privacy rights" trump every other concern (even concern over whether a student lives or dies), then the indifference of the colleges to the welfare of their students makes sense. But it's curious that for so many years (til the '60s, in fact), nobody would have imagined young adults living off their parents had such rights.