There's no point in comparing the EU and the US as wholes. What matters is the density of a particular urban core. Many American cities have dense cores. Beyond the obvious big three off the top of my head some cities I have visited that could support some closed streets in the core: DC, SF, Baltimore, Philly, New Orleans, Minneapolis, Portland, Honolulu, Charleston, Dallas. No one is suggesting changing everything everywhere, but large cities do exist in every corner of the country.