Part of it is the spread-out car culture of the US, where virtually the entire country is so car-centric that walking anywhere is prohibitively difficult and/or risky. It's kinda self-reinforcing - walking is awkward and discouraged, so almost nobody does it, so there's no incentive to fix any of the walkability problems, like missing or neglected sidewalks, bad traffic light behavior, huge parking lots everywhere to cross, etc.
But the bigger part IMHO is the stranger-danger paranoia. Google the free-range parenting movement here for a few disturbing stories. The media hypes up stories about rare child molesters who kidnap children off the streets for better viewership numbers, then people start to think that kidnappers are everywhere and you don't dare take your eyes off a kid for a minute. This makes its way into policy and expectations. Add that into the common viewpoint here that if you have a child, they must be the most important thing in your life and your entire life must be centered around improving the kid's life. You get a society where letting a child, or even teenager, walk to school seems so outrageously dangerous and irresponsible that kids may sometimes be picked up by the police and returned to their parents even though nothing is wrong, and the parents may sometimes even be investigated themselves for their supposedly wildly irresponsible behavior.