I'm just commenting on why people consider having a car a necessity in the U.S.; the reasons for it are more complex. Population density is one reason, but infrastructure planning and preferences is a big part of it. Most Americans live in the populated parts of the country in or near a city, not in rural Nevada or Nebraska, so those huge expanses on their own don't make public-transit impractical (they do complicate
intercity rail, but that's a different issue).
Urban sprawl does make things more difficult, but doing better than the current state would still be quite possible in many cities. For example, taking the two most recent cities I've lived in, Houston and Copenhagen have closer population densities than people may think in the majority of their commute basin. If you draw a circle with radius roughly 30 km from each city's downtown, both have a density of about 1,000/km^2. Yet Copenhagen has 5 commuter rail lines, a subway, and an extensive 24-hour bus network, while Houston has 3 light-rail lines and a pretty bad bus network. So of course it's harder to get around in Houston without a car, even in the dense parts (for example, it's a huge pain to get from a high-rise condo in the Galleria area to an office building downtown, even though these are both dense areas and only about 15km apart). Houston is slowly improving in the past 10 years, though.