How would you have any idea if it's "significantly higher in the suburbs" ?

Subway and mass transit systems cost billions of dollars to build and maintain. Power, water and sewer infrastructure; policing and fire protection; subsidized insurance and natural disaster response; parking, health and sanitation -- These and others are the massive costs of high density urban living. This is a scalability issue. And my understanding is that it's difficult and expensive to build these things to scale to urban densities.

I think you're being myopic about the costs of cul de sacs and interstate congestion.