Because there's not 7.5 billion acres of arable land that's also got good, stable water access. Also, because someone living in a major city produces dramatica

Because there's not 7.5 billion acres of arable land that's also got good, stable water access. Also, because someone living in a major city produces dramatically less carbon due to economies of scale, and your idea would result in an incredibly low density, offsetting any gains to food shipping cost.