Public transport is seen as being really sketchy and dangerous in America, so the idea of letting your kid ride it unattended is seen as neglectful in many areas. There’s even a whole Simpsons episode about it, where Homer is his classic idiot self, telling his kid to ride the bus like that’s okay, and spends the episode trying to find her when she inevitably ends up somewhere she’s not supposed to be. At the same time, parents are expected to be at work by the time the kids are going to school, so it’s not common to have the parents drive them, either. Thus, the ubiquity of the school bus.