Many parents don't trust their 5 year to get on the bus everyday. High school can surely be moved back for areas with 100% bus availability (my district has not had buses since covid), but elementary school would be a much tougher sell.
That's a fair point for kindergarteners, but by 1st or 2nd grade kids in my district walked to the bus stop on the corner all by themselves just fine. Also, my district had elementary school start the earliest (and middle/high school would start later) which for some reason is uncommon but makes a lot of sense for a whole bunch of reasons and would seem to mostly solve this problem for parents who need to walk their kindergartener to the bus stop before work.
People have faced legal issues for letting their 6 year old safely roam in my area. That may be crazy, but it's the reality in much of the US.