It's a symptom of our increasingly car-dependent infrastructure imo. You feel much more exposed in the middle of parking lots or walking along neighborhood roads without sidewalks, or trees and buildings providing cover, and god forbid you end up footing the main arterial roads we have to take to access any commerce. More of our lives are being lived in these "in-between" areas, making us feel less safe in general. Houses being further apart and less people going out on foot means there are less eyes incidentally looking out if anything were to go wrong, and kids have to go further to get anywhere safe.

All of this makes adults feel less comfortable letting kids go out on their own, and cast judgement (and sometimes call the police, who sometimes take legal action!) upon anyone that does. Plus the monoculture of suburbia makes people much more susceptible to fear mongering by mainstream media, not to mention the unprecedented access to that media provided by the internet.

Anyways, here's a great video from Not Just Bikes on the topic, Why We Wont Raise Our Kids in Suburbia