Because of how each state handles education separately, and the presence of private schools, in the US you have a wide variety of high school outcomes. From kids that aren’t literate, to kids who could probably earn a bachelors in less than a year, if their high school education was considered in a more holistic fashion. Therefore, university programs that are set up to cater to the largest population possible necessarily aim for the lowest common denominator. And of course there is always the perverse incentive that colleges and universities are paid by the credit, so reducing course loads would probably never make it past the bean counters.