Apples and oranges. Cities can't sustain themselves without agriculture and products from outside of the city. If I never go into a city, I will never use their public transportation. If you never leave a city, you will still use public roads in order to eat, even if you never actually step foot outside.
The countryside also can't sustain itself at its current standard of living without goods and services produced in cities.
The whole point of infrastructure is that it benefits society as a whole. This is just as true of city infrastructure as it is infrastructure in the countryside.
(Not to mention there's a whole lot of countryside that doesn't produce food...or much of anything, really, other than a place for misanthropic people to retire to.)
