I think you underestimate the amount of bad health environments there are in cities (anecdotal evidence only). I would not call most cities or working environments anywhere close to sterile.
To be honest, nobody is talking about very deadly exposure to pesticides in farmers populations. There actually have been papers published on that. While living outside a big city might be on average better for you, being a farmer is probably not.
Actually: lots of people bring a lot of diseases with them, so it you judge by germs, I would argue a city with buses, train stations, airports and harbors would be worse than any soil outside of it. Though it says nothing about food, can't argue there. Though (from experience) "farmers" tend not to be too broad in their nutritional uptake and eat mostly the same where I come from as people in the cities. Might be different in the states though.
Its immunity before you're exposed that at issue. How did you grow up? What routine exposures did you experience?
I grew up muddy and dirty, eating anything I found in nature that was mostly edible (apples, ground cherries, berries, chewing bark and grass), as well as organic beef and pork (before we knew what that meant) and garden vegetables. That was closer to historical experience.