The government sets the hours of the schools, which in turn drives the schedules of a fair chunk of society.

Even that isn't very hard and fast, states can set their own school hours, but in nearly every case there is only an earliest time and an overall yearly hours of instruction. They can start at different times or change it everyday if they wanted, that is controlled by either the school or possibly the local government.

Schools choose to use similar times because they think that is what parents want.