I'm not sure the average population density across the entire country is that relevant - it doesn't tell you anything about the distribution of population.
The US is actually rather an urban country - 82% live in urban areas which is more than Italy (75%):
https://en.wikipedia.org/wiki/Urbanization_by_country
Edit: Also noticed that Australia is 86% and Canada 81%!
I mean, Berlin has 3.7 million people in a city of 890 sq km. Melbourne has about 5 million people in a city of almost 10,000 km. They're incomparable, and so the urban population of Australia and Germany are incomparable. (You can play with the definitions as much as you like but you'll never be able to compare arbitrary Australian and German cities.)
Half of the US population lives in the suburbs (see above posters). What we would recognize as a real urban environment only accounts for less than a third of the US population (98M people).
I did see a figure of about 55% of everyone in England and Wales living in suburbs - but I suspect our suburbs look pretty dense to someone from Houston or indeed Achiltibuie!