>You can build dense cities in the US all you want
No you can't. Dense cities have been outlawed by misguided zoning codes and traffic engineering standards.
This was celebrated by the architects; Frank Lloyd Wright's Broadacre City celebrated the ideal of every home sitting on an acre of land. Contrast that with the Tuscan town of Pienza, Italy, which sits on a bit over 11 acres of land and which contains over 2,000 residents. Just looking at the numbers, Americans would think it must be some horrible slum. But it's such a fabulous place that it attracts close to a million visitors a year, if I recall correctly. It would be completely illegal to build Pienza in countless American towns and cities, and on many counts. In reality, Pienza would probably violate close to 90% of the conditions in a conventional Euclidian code.