People drive into their garages, go inside from the garage, and rarely stray outside after that. And if they do, the only place for them to go is their fenced-off back yards.
As a result, many subdivision-dwellers don't know their neighbors.
And, of course, there's the "ninny state" effect at work, too. Parents want to protect their kids from all sorts of monsters -- some that are real, of course, but many that are not.
Yes, there’s a lot of truth in what you said. Another reason might be that people are having fewer kids these days, and “helicopter-ing” over them a lot more.
Honestly, I don’t want to know my particular neighbors. We have a chain link fence in our backyard. My next door neighbor is always asking, “Why aren’t you out more?” Just because I’m not out on his schedule does not mean I’m not out.