Trains are impractical and the tracks take up so much space

San Francisco population: 815,201

…m.wikipedia.org/…/Denver_International_Airport

“The 52.4 square miles (136 km2; 33,500 acres) of land occupied by the airport is more than one and a half times the size of Manhattan (33.6 square miles or 87 square kilometres). DIA is larger in land area (excluding water) than the US cities of Boston, Massachusetts and San Francisco, California. DIA occupies the largest amount of commercial airport land area in North America, by a great extent.”