City dwellers also buy things manufactured in rural areas. Most of them expect a nationwide transportation network that isn't just interstate highways and gas stations. In addition to food, there is all the resource extraction that needs workers, who have families, and need healthcare etc. Basically a lot of people living in rural areas are either doing things that, in part, support people who live in more urban areas or they're supporting supporting those people.

Cities eat up tax dollars too. Boston's Big Dig was basically a $10 billion or so gift to Boston from, not only Western Massachusetts taxpayers but the rest of the country. (The Speaker of the House was from the Boston area.)