In large part it's an unfortunate consequence of how academia works; but I think in large part, what you're asking for is happening: the proportion of instructors who are lecturers and adjuncts, rather than tenure-track/tenured professors, has steadily increased over the years [1].
Meanwhile, tenure-track/tenured professors tend to focus primarily on teaching upper-level classes, which suits them infinitely better. The students they get are much more knowledgeable, which frees profs from worrying too much about background knowledge; by sheer virtue of selection bias, students that aren't interested have been filtered out; and the material is much closer to the work the profs themselves actually do.
Meanwhile, tenure-track/tenured professors tend to focus primarily on teaching upper-level classes, which suits them infinitely better. The students they get are much more knowledgeable, which frees profs from worrying too much about background knowledge; by sheer virtue of selection bias, students that aren't interested have been filtered out; and the material is much closer to the work the profs themselves actually do.
[1] https://www.theatlantic.com/business/archive/2013/04/the-eve...