It's true. The big reason that kids don't go out and play anymore is that there are no kids outside playing anymore. I live in a neighbourhood where I'm only a few blocks from 2 elementary schools and I never see any kids out playing.
A product of helicopter parenting in many respects. New parents don't tend to like the idea of letting their kids running around the neighborhood. The way those well off enough like to reconcile this with balanced lifestyle is overschedule activities for their kids. They get ze dezignated social time at hockey, soccer, karate every other night. Busy-ing a kid's life has its own pitfalls.
Local business owner was telling me that police showed up at her business with her 9 year old because the 9 year old was "out on her own". He was walking from the local business to their house, about ... 8 blocks from the business. It's a smallish town (40,000 population, but in this 'old town district', maybe 2-3000?). Police brought the kid back to the business and chastised the business owner, intimating that letting her kid walk home unattended was in violation of some law. She asked for the specific law, which pissed off the officer to no end (because there isn't one here).
There is safety in numbers; parents will let their kids go out to play when there are other kids around. And depending on where you go in the country, you'll see groups of kids playing or you won't. But it's self feeding, as fewer kids play outside, fewer parents let their kids play outside. My kid has no reason to go outside if there is nobody out there.
