Or virtually all of the Persian Gulf. Some of the fastest-growing populations, in what's empty desert without even sufficient water for cities let alone agriculture.

The economist's answer is that these regions have economic outputs (financial wizardry, advanced manufacturing, tourism, oil) which the rest of the world wants, and the rest of the world (which in the case of the Atlantic seaboard is largely the US's own inland agricultural regions -- breadbasket to the world) is willing to exchange its agricultural produce for those goods.

There's a difference between economic self-sufficiency and ecological self-sufficiency. Arguments can be made over which is preferable. Comparative advantage means that the economic argument leads to greater overall population and productivity, sustainability means the ecological argument leads to greater sustainability.

There's also that little matter that agriculture has largely transformed into a factory system for converting petrochemical energy to food energy via modest sunlight inputs.