America does not have an even population distribution. East of the Mississippi is similar to Europe in population density, as is the far west coast (within ~100 miles of the Pacific). There is a lot of land between the Mississippi river and the Pacific Ocean, plus Alaska that few people live on that brings density down. However if you just focus on where people live density is high enough.

Even the sprawling suburbs are dense enough to support greats transit - but since they don't have great transit everyone drives creating a death spiral that is hard to break out of.