I think that's a big part of it, another part is the false idea that our kids are not safe outdoors without parental oversight. Kids cannot just go out and ride their bike to the river and fish, or go the playground and meet up with a dozen other kids to play ball on their own. For a kid these days if a parent doesn't organize it, take you there and sit and wait for you to finish you can't go.
Yeah, we've all read the stories of parents getting arrested for letting kids walk to the park, while in our day kids went all over the place. I remember walking around DC when I was 5, or taking a bus downtown a few years later. Or walking with my brother to and from church, or even a swimmig pool in Maryland. We moved to Arkansas when I was 9, and all the kids walked everywhere there, too. These days parents get arrested. I even had a school threaten to call the cops because DW got stuck in traffic coming to pick up the kids.
Not just in the US. Someone did a study of 4 generations in the UK, all living in the same north side neighborhood in the same town, and how far they could travel on their own.
When great-grandpa was about 6, he could walk several miles to the south end of town without supervision.
Grandpa, at the same age, would travel about half that distance, going to the center of town.
Mother could walk a few blocks in her neighborhood.
Son is NEVER out of direct line of sight of an adult.