It’s not, though. Starting in the '30s, car-dependency became a deliberate policy choice. Zoning codes and rules for FHA-backed home loans changed in such a way as to heavily favor single-family homes and businesses with extensive on-site parking, or in many cases prohibit dense traditional development (both residential and commercial) entirely.

On top of that, shortly thereafter (after WWII) Eisenhower pushed through massive public investment in the Interstate Highway System, to the extreme detriment of the railroads.

Make no mistake: car-dependency is not the default; it only seems that way because it has become entrenched over the decades via massive deliberate effort and a continuing firehose of taxpayer cash. If the rules maintaining it were abolished today, the development patterns would start to revert back to traditional (dense) development.