I don't think there's any one factor that can explain the whole thing. Food surpluses, the addictive nature of sweet things, a culture that pushes convenience ... Take your pick.
For what it's worth, though--after two years as a night checker in the heart of corn country, I got to know a lot of my customers pretty well. And the ones who were healthier were the ones who cooked their own food rather than relying on the processed stuff. It wasn't necessarily gourmet-level cooking, either, just basic ingredient combinations, but even the fat ones who cooked were less fat than the ones who came up to the checkout with a cart full of Totino's Party Pizzas.
(Sidebar: I fucking hate Totino's Party Pizzas. They are the tell-tale sign of someone who's given up on life.)
I think if you teach kids to cook, to experiment with their palates and feel the satisfaction of having made something, they can develop a better relationship with food. Obviously this isn't a guarantee (anyone remember Simply Sara?) but it'll at least teach them to think of food as something other than instant satisfaction in a plastic wrapper.
And make them some homemade bread. Four ingredients, don't even need to knead it, and it makes better sandwiches and doesn't disappear as fast as that fucking sponge the supermarket calls bread. Corn syrup does not belong in my sandwich, dammit!