if you're from the usa, your grandparents could buy corn oil and margarine, and didn't see them as luxury items; but their generation didn't make a diet out of it

conventional convenience foods like mcdonalds hamburgers, coca-cola, and twinkies are about 3, 0.5, and 4 kilocalories per gram; olive oil and butter are 9. healthier foods like whole-wheat bread, eggs, eggplant, and fatty steak are 2.5, 1.5, 0.4, and 3.5 calories per gram. calorie density is not what makes eggs healthier than coca-cola

your hypothesis would also imply that people on low-carbohydrate diets consisting of fatty meat and cheese would rapidly gain weight, but instead they lose it

in case it wasn't clear, i think the explanation that the obesity pandemic is due to a sudden decline in willpower is unlikely