i thiiink it's to help everyone see & hear better, like in ancient Greek theatres
this is all guesswork but, i'm guessing that since university education historically (and in a lot of places to this day) is more a thing of the rich they actually put some thought into the design of the lecture halls. And for the education of the poors that's simply made to condition them to work in factories they just put some tables and chairs in a room and called it a day - and since then the design stuck
Your guesswork is doing a bit too much there. Rich schools also have flat classrooms for smaller groups, e.g 30ish.
The reasons for stadium seating is for size, and that's true for most schools including community colleges (and even vocational schools). Usually it's used for classes everyone has to take, like a pre-req. High schools aren't standardized in the same way, so you generally wouldn't have a class of 80. High schoolers need more one on one anyway, and teachers require less specialized knowledge, so the numbers just work better that way.
our uni had mandatory chemistry for all engineering degrees, even industrial economics and computer engineering, so naturally a lot of people had to re-take it. the registrations weren't done processing for weeks, and during that time you had to assume you were in, or you'd be hopelessly behind. that lecture hall was filled to the brim the first few weeks every semester. people were sitting in the aisles, on the stairs, in the window sills, two to a chair... it was insanity. i really hope they fixed that.
Back for me because I have a pretty disgusted, annoyed resting face. If I sit too close I'm focusing too much on keeping a pleasant listening face that I don't pay enough attention to the material and give myself a tension headache.
Had three classes (5 hours) back to back in the same room one semester. I ended up switching seats to break up the monotony. I started at nerd and slowly progressed to the back for sleep.
I just tried to get to the front, even beeing 20 min late.
I had to update my glasses while in university for each semester.
So I would sit at any of these seats.
When friends reserved one seat in the back, I would take it but tell them that next time I have to sit in the front.
In my country students aren't judging people at such minor interactions.