Limited application to STEM subjects. Yeah, CS and EE can finagle that nowadays (wasn't true for CS when I might have gone that route in the early '80s), but the sciences require lab work that you generally aren't going to be able to do, let alone afford, anywhere else but a school. And there are serious safety issues as well, plus the minor detail for them that an undergraduate degree is just a requirement to get your (paid for) terminal Ph.D., unless you're willing to end up being a lab tech.