Urban vs. rural farming

So last time I was in China (more than a year ago, I assure you) I noticed what appeared to be farmland from the balcony of an apartment building more than a dozen stories high. This apartment building wasn't exactly in the middle of town, but it was along the outskirts of a major city, with a major supermarket nearby, so it was clearly in an at least somewhat urban setting.

This is weird because from debates over big cities vs small towns, you always see advocates of the latter say things like "we farm your crops!"

So I'm wondering; is this done for medical reasons, or just economic ones? Is urban land too polluted for any first-world democracy to find it acceptable to get our food from it, (China isn't known for food safety as recent events have shown) is the crowding considered a pathogen-spread risk, or is it just a matter of first-world urban land values being too expensive for farming? Or is there something else I'm missing?