to answer your question, there is no reason other than america’s fetishization of the protestant work ethic.
schools don’t need to be a joyless labor camp. you already have to wake up far too early and be there all day, can’t you at least be on your phone? maybe give kids a break? everyone has stress in their lives, my anxiety started in 5th grade, maybe i don’t have the mental capacity for 7 fucking classes today and i check out after 5. just like no honest person pays full rapt attention every minute of their jobs.
in college you can basically be on your phone during class and i remember just as much from college as i do grade school. either way, you have to study for the exams. if you aren’t gonna pay attention, there are plenty of ways to do so. being forced to listen doesn’t necessarily increase absorption.