1) Obesity isn't a disease of the rich. So the "cheapness" doesn't seem to be an obvious factor. This is in contrast to the 1600s and 1700s when obesity and gout were diseases of the rich in Europe.
2) In poor areas, especially the third world, we are starting to see a pattern obese mothers and malnourished children. This is bizarre. The usual pattern is to see malnourished mothers and malnourished children.
3) We can simulate the syndrome with insulin injections, which seems to suggest at least one hormonal pathway driving overeating (rather than simple psychology).
4) Women get fat after menopause (hence the estrogen supplementation industry, which turned out to be giving them cancer). Everybody starts to put on pounds at middle age. Kids seem to start eating a lot at puberty, but generally grow up rather than around. It seems like the whole system is much more complicated than "cheap, tasty food."