It's beneficial for the rest of the US to find solutions that make high density cities successful. Such locations tend to be both your cultural and economic centers, in particular due to the network effects of putting so many people in close proximity to each other. (The Internet helps but hasn't proven to be a substitute quite yet.)

Additionally, dense urban areas tend to have the smallest environmental impact per capita (lower carbon footprint in particular), and help prevent the problem of urban sprawl invading rural areas. Nobody wants to live in a suburb that covers the whole country.