Let's experiment with that statement. Perhaps what I'm hypothesizing here is new, perhaps it isn't.

Don't take this as a position to argue with. I really don't have a dog in this fight.

Suppose that people have always been after a certain feeling of fullness, and this feeling is largely a product of evolution. It's also very emotion-driven, not purely physical.

When humans feel full, they stop eating. Of course it doesn't work perfectly, since we've always had fat people. It does happen to work for me and my extended family, since none of us have ever been obese.

Suppose that animal fats, being highly calorie-dense, give a feeling both of being full and being emotionally satisfying. Every chef will tell you that people love fat.

Now, if the food industry and nutrition scientists removed a lot of the animal fats, then both people's physical and emotional needs increased, and many searched for sugars and carbs to compensate. They are a poor substitute, though.

If this were true, then humans are getting more calories than they were when animal fats were a bigger part of the diet. It's not that animal fats are good for you as much as "if you don't consume them, you'll consume other things that are even worse."

Of course this is not a single factor to explain it all. A more sedentary lifestyle is also a big, big part of it.