The design of the physical environment is in no small part responsible for this. Don't believe me go to Europe and Japan where kids are still allowed to walk and bike to school, to friends places and just in general play and exist outside unattended to a much higher degree than in the US. Because in the US they would be roadkill, and even if they somehow survived outside of a car, car centric design means there's now…
The biggest cities of Europe (Paris, Amsterdam, etc.) are pretty historical and thus are walkable. However, where I live - Finland that is - much of the country has been built in the past century and is designed for cars. However, in the past decade or two the trend has changed again and the very new neighbourhoods again feel like 100+ year old neighbourhoods in terms of walkability.