Oil (and fat) is crazy because it's so calorie dense. Not only is there 9 calories per gram (vs carbohydrates which are only 4 calories per gram) it's in a tiny package. Not that sugar is healthy, but for scale: a tablespoon of sugar has less than 1/2 the calories of a tablespoon of oil.

Great video about oil here https://www.youtube.com/watch?v=OGGQxJLuVjg

Now what is the average caloric expenditure for the "average American" over the same period? I bet that in addition to the average caloric consumption going up 800/day, the average caloric expenditure went down due to a lot less people working in physical jobs now compared to 50 years ago...

It seems calories-in/calories-out would be a much simpler explanation for the "obesity epidemic" compared to eating high/low fat high/low carbs etc. etc.

I think you're right about people moving around less in general, and it no doubt has played a part.

But there are also studies which show that even if calories in/out remains the same between all diets, the diet which is less processed food and lower fat food does better.

E.g. https://www.nature.com/nutd/journal/v7/n3/pdf/nutd20173a.pdf

I'm normally a big fan of Occam's Razor, but this is one of those questions that demonstrate why it's a rule-of-thumb, rather than a law. As the study that @transmorpher linked to shows, there appears to be a serious degree of homeostasis in the human metabolism. In short, the simple explanations just don't explain the observations.