In broad swaths of the midwest, the outside isn't especially attractive, if it's not outright ugly. Minimal elevation change anywhere in sight, little visible rock outside road cuts, very few—and all unimpressive—waterfalls, slow, muddy, gross streams and rivers. And the landscape may well be like that for
hundreds of miles in any direction.
Then, on top of it, the weather's miserable 9 months out of the year. Oppressively hot & humid, or bitter cold. You're inside, you're in a pool/manmade-lake, or you're eagerly counting the minutes until you can get back to one of those places. Because of the first paragraph, incentive to brave the elements and go outside anyway, is practically zero.
IOW our natural environments and climates also discourage outdoor activity, pretty damn effectively.
[EDIT] Oh, and regarding this:
> Obviously it's not profitable to own a healthy restaurant out there, but why? Is it a lack of healthy eating education? Is it a "harder" life and junk food is a reward? Or is it cost?
It's a combination of cost and population density, mainly. Food culture plays in, but it's a feedback-loop sort of situation, not the case that food culture's purely driving the problem. Restaurants that optimize to avoid ingredients that spoil fast can charge less. Healthy ingredients tend to spoil faster than unhealthy ones (not universally true, but broadly so, especially for fruits and veggies). Higher prices (relative to local competition) mean a smaller customer base, which increases the population density required to support a restaurant.