IF you can buy an acre, please definitely do so. That will do more to help than almost anything else. On the other hand, the typical 2.5 kid family in Suburbia that drives even to the grocery store? They're better off moving to the city.

That's also an option, but most people living outside core urban areas don't do that, and adopting such a lifestyle probably is the harder choice for many.

Instead, they drive a lot, because everything (stores, work) is far away, and the infrastructure created for them is relatively inefficient. Very few people and regions are supplied locally, and not by large-scale, large-distance supply chains, rural or urban.

I would be quite surprised if growing your own food at small scale is more environmentally friendly than living in an urban area and consuming food that was produced and transported at massive scale.

Depends on what kind of growing your doing. There are a lot of agriculture styles that use very little resources and are economically affordable. If you're doing row-style monoculture, then you really can't compete with those economies of scale (and subsidies). But permaculture has huge yields for the amount of input it requires. Cultivating fruit trees and perennials in a plant "guild" can produce resilient, long term food sources with little upkeep. You could easily grow enough food for yourself on an acre.

Why is this being downvoted?

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.