I believe that time and energy are what many Americans are running in too short of supply on to transition to more wholesome diets. Most have so much on their plates to deal with already that the idea of thinking about additional things they technically don't have to is unappealing, even if it's at the cost of their own health. It's why ultraprocessed foods became so common in the first place.

It's just another microcosm of a culture that's obsessed with productivity and has little resembling proper work-life balance.