Why talk about cities' "accessibility zones" in area and not population? Sure, America has 1,160 accessible km²s vs Europe's 430, but that area is far less dens

Why talk about cities' "accessibility zones" in area and not population? Sure, America has 1,160 accessible km²s vs Europe's 430, but that area is far less dense. The average population density of the US is just 94 people per sq mile [1], while Germany stands at 619 pp/mi² [2]. [1]: https://css.umich.edu/publications/factsheets/built-environment/us-cities-factsheet https://css.umich.edu/publications/factsheets/built-environm... [2]: https://www.worldometers.info/world-population/germany-population/ https://www.worldometers.info/world-population/germany-popul...