You're missing two things, one for each age group:
For the younger kids, each hour earlier they start is an hour earlier they finish, which means parents need to either finish work earlier (not possible for most people), or pay for an additional hour of after-school child care (expensive). Having free child care to cover the gap between school finishing and work finishing is extremely rare, and the acceptable age to leave your child unattended after school is going up due to over protective parenting (for example, I was a latchkey kid starting in the 3rd grade, but nowadays that seems almost unheard of until middle school).
For the older kids, starting later means sports / extracurriculars finish later, potentially after dark or after the parents are done working and expect kids to be coming home.
For the younger kids, each hour earlier they start is an hour earlier they finish, which means parents need to either finish work earlier (not possible for most people), or pay for an additional hour of after-school child care (expensive). Having free child care to cover the gap between school finishing and work finishing is extremely rare, and the acceptable age to leave your child unattended after school is going up due to over protective parenting (for example, I was a latchkey kid starting in the 3rd grade, but nowadays that seems almost unheard of until middle school).
For the older kids, starting later means sports / extracurriculars finish later, potentially after dark or after the parents are done working and expect kids to be coming home.