It must be culture, at least in areas where walking to school is pretty convenient. Anecdote: When I was growing up, I walked to elementary school, all of five or six neighborhood blocks. The one kid who got driven to school, got teased for it.

Today, same school, same neighborhood, is a traffic jam every morning, as the kids are all driven to school. I could very well imagine parents saying that it's no longer safe to walk, due to the traffic. :-(