In my own experience, first-year GPA in college was a cakewalk. I had straight As until Junior year, then things got a bit more demanding and I was caught off-guard because college had been pretty easy up to that point. (Large state university).
My experience was the opposite. First two years were a blur, I was in the trenches juggling calculus and organic chemistry and physics along with my major work and the extra busywork courses they throw on top of all that too. By the time I got to my upper division work in the third year it was like grades didn't matter at all. There's be no more assignments or quizzes, maybe two exams to determine your entire grade for the course, that would be pretty easy to get an A on if you attended class and didn't sleep.
It could just be that professors don't want to invest a lot of time designing a bevy of coursework. In the weed out classes they are all lecturing off a textbook, which provides for them slides to use and a general schedule of topics, as well as banks of questions they could pull from and permutate and reuse. Its a lot of time saved for sure. In the upper division, there is no textbook or anyone designing any course, they might literally have a sentence description of what the course is pitched about in the catalog then they take it from there. The professor has to come up with what to talk about from their field for an hour twice a week, and sitting there drafting up those slides is enough work if they don't have any from last year to use, considering they are usually also a full time researcher on top of this.
