As I mentioned in another comment, I don't have any argument as to why. Just how it was when I was in school so thats what I'm used to.
But I also mentioned that there are often thousands of students all trying to take one course. And the schools simply don't have the space to fit all of them in one session since I believe the rules are basically that it needs to be held in a lecture hall big enough to fit every enrolled student, and teachers don't have the time to teach 4 different sessions by themselves on top of their other duties. Maybe class sizes are just smaller elsewhere, but where I went to school it was not unheard of to have multiple thousands of students needing to take one class that was required for practically every STEM major in a given semester.
But I also mentioned that there are often thousands of students all trying to take one course. And the schools simply don't have the space to fit all of them in one session since I believe the rules are basically that it needs to be held in a lecture hall big enough to fit every enrolled student, and teachers don't have the time to teach 4 different sessions by themselves on top of their other duties. Maybe class sizes are just smaller elsewhere, but where I went to school it was not unheard of to have multiple thousands of students needing to take one class that was required for practically every STEM major in a given semester.