Massachusetts, for example, has a population density of about 340 / sq km
And the cities within it are much more highly populated still, with population centers at the East and west ends. The center is relatively less populated.
The middle is agricultural land. You have to consider rural areas akin to industrial areas or open air factories. Because acres under industry is not suitable for residences.
Agricultural/rural land doesn't need public transportation. The actual space within the US that merits it is a tiny fraction of the country, a much smaller fraction than in more densely populated European countries.