Also so students are at school before parents need to be at work. School is child care for a lot of parents.
School buses still exist too. Kids in rural areas can stand at the end of their driveway and board a bus without parental supervision, provided their parents aren't totally paranoid.
It’s one thing to let your kids walk to and from school when you know there are lots of other parents around. It’s entirely another thing when you know the place is a ghost town until all the parents get off work and come back home.