The real reason why they don't is that parents need to get to work. School starts early so that parents can drop their kids off and then head to work on time.
Idk, man. I used to have to get up at 6:00 am every morning so that I could be at the school bus stop by 7:10 am. Now all I see is a huge line of cars waiting to drop kids off or pick them up. I can't even remember the last time I saw a school bus for kids younger than high school.