Density is measured as of number of people per square mile. Since you can’t squeeze too many people/houses into a flat area, you have to build up into taller buildings. The US isn’t uniform in structural need and regulations (ground can’t support the same weight, seismic structural safety gets disproportionately more expensive the higher up or down you go, other expensive features become mandatory over certain heights, etc) so you couldn’t even have the same density in other parts of the country.