National population density is irrelevant. US population is just as urban as Europe. The lower USA average mostly reflects the southwest deserts and Alaska where very little rural population exists.
The low density of car-oriented US suburbs was mandated by socialist central land use planning from Washington and the motorcar industry that wanted to require Americans to each own a car. Standards were written centrally and local governments were required to make them law with grants, national bank regulations that required them for lending, and professional standards organizations that made civil engineers agree to them for licensure.
The more market driven land use in European cities promotes walking and transit more than personal motoring.
The low density of car-oriented US suburbs was mandated by socialist central land use planning from Washington and the motorcar industry that wanted to require Americans to each own a car. Standards were written centrally and local governments were required to make them law with grants, national bank regulations that required them for lending, and professional standards organizations that made civil engineers agree to them for licensure.
The more market driven land use in European cities promotes walking and transit more than personal motoring.