Most farmers don't produce most of their own food.

The modern world is built on specialization: most farmers have specialized in some direction or another, and the ones who haven't usually make less money. At best, a farmer with livestock will occasionally take a pig or cow off to be butchered (since butchering is another specialty entirely, needing specialized equipment to do well), or a farmer growing corn will plant a few rows of sweet corn in a field near his house, or a farmer growing produce -- usually just a half dozen varieties -- will keep back a little for himself.

But the calories a farmer will produce for himself are generally a small part of his intake. In the end, it's a better use of his time to focus on producing as much of a handful of crops as possible and buy what he needs from a grocery like the rest of us.