Cars and building for car infrastructure is part of it. Another part, I think, is the decline in neighborhood communities. By that I mean the social pressure to get to know/socialize with your neighbors, through everything from block parties to shared church membership. When kids go “wandering the neighborhood” they were never far from one of the member’s houses, or at least a familiar neighbor who would notice them and keep an eye out.

Which also goes back to car infrastructure. If you need to drive everywhere for any and all errands/activities, you won’t interact with people in nearby houses, you wont see neighbors at the local bar or small grocery store.