The great majority of development in American cities over the last 75 years has been single-use, with neighborhoods of exclusively single-family houses separated from nearby commercial strips with big parking lots along wide roads.

The downtown/center of older cities may still have mixed use, and there have been changes happening in recent years to allow/build more apartments and mixed use areas, but, generally outside of the densest parts of the largest cities commercial and residential areas are required to be separate, with personal cars as the primary/only way to get between them.

This has been a bit of a self-reinforcing phenomenon, IMO, as car-first infrastructure puts people at the mercy of traffic congestion, and means that any apartment building or business in their vicinity will result in more cars passing through, more congestion, more competition for parking, as well as the presence of the large parking lots that cities mandate for any new construction, which themselves make it unpleasant to get around in any other way.