One factor may have to do with birth rates and construction. I grew up in a neighborhood that was all built up within the span of a few years, and populated by young families, in the early 60s. There were kids all over the place. Anybody who wanted to play would just go out and holler, and they'd have a few other kids almost instantly.

Where my wife and I raised our kids, there was one neighbor with kids, and that's it.

Also, kids are more occupied now. "Back in my day" elementary school kids didn't have homework, and it was pretty minimal even through high school. My kids had homework starting in first grade. Naturally you want it to get done early while the kids are still awake, but this cuts into the prime hours for play. We should simply have revolted against it. But that's hindsight.