Well when you realize we treat school as glorified babysitting and not just education, part of the reason becomes more obvious. Parents work 40 hours so we need kids in school roughly that length of time. Especially when both parents have to work to afford to live.
We need to uplift a lot about the entire system for it to work.
Also ppl are obsessed with high school sports so that's another reason why they start high school so early (my high school started at 7:45 AM), so there's time after for sports practice.