The inevitable result of three things:
1. an absurdly car-friendly society that prioritizes cars well above everyone else - basically, parents have to pick up their children in cars because there is no reasonable pedestrian infrastructure to allow them a safe walking home.
2. a truckload of busybodies ("Karens") that call CPS on children wandering alone - Utah had to pass a law in 2018 [1] to cut down on that crap and other US states are following suit. Unfortunately, in states without that explicit carve-out, schools can be cited for neglect as well.
3. US healthcare being absolutely rotten to the core, which means that should a child get injured on the way from the school home, their health insurance will try to pass the bucket down to somewhere else, and the first obvious thing is to attempt to sue the school because they were the last ones in custody of the child. As a result, schools introduce such policies because the second the child is in custody of the parent, the school is no longer liable for anything and the insurance can go and stuff its claims.
[1] https://www.npr.org/2018/04/01/598630200/utah-passes-free-ra...