It's probably a combination of all your questions. Simpler fee structure that takes a standardized part of young adulthood that provides a standardized amount o

It's probably a combination of all your questions. Simpler fee structure that takes a standardized part of young adulthood that provides a standardized amount of expertise, leading to "an undergraduate degree" denoting a certain amount of breadth and depth. It's about what a typical college student can learn in four years. Some fields like law and medicine require more time to get practically useful, like 7ish years for law, which we can break down into units of 4 + 3, an undergraduate degree and a law doctorate (JD). Or medicine, which takes like 15 years, and is split into 4 + 4 + 3-5 + 1-4, an undergrad degree, a medical doctorate (MD), a residency, and a fellowship. Maybe splitting it into these points helps standardize those and provide convenient points where paths split.