It has been noted that dietary recommendations often coincide with the production mix of the the national agricultural sector. So food pyramids and whatever often has more to to with trade policy than with physiology, and of as late environmental policy. Just see that controversy around LCHF; what that comes down to is that a world on a LCHF diet would be even less sustainable that the one we have today, and would cause spikes in grain prices (cattle require a lot of caloric input per caloric output, and hence a meet eating world is a more calorie intensive world) and thus mass starvation in poorer countries as food would be sucked out of them and into the industrialized world in order to feed livestock. But you can't tell that to the sheeple. If they believed that LCHF was more healthy to them they would eat it and compartmentalize the effects. So you have to sell them on the idea that LCHF, or real food as its proponents call it, is somehow bad for them even though the shit they are eating today is actually worse.
But regarding the subject of the thread, I would suspect that these habits of unhealthy eating are more representative of the lower classes. A low ability to take in and process information and, above all, very high discounting factors, make them suckers for immediate gratification through white flour, fat, sugar, and salt. Interestingly they are all vegetable based and thus contributes to keeping the kids off meat. Problem solved, so to speak.