I mean, it's ultimately that there's a massive industry that is incredibly successful at making us eat a lot of food that is bad for us.
The differences in exercise doesn't really make the kind of difference we're seeing.
It's because advancements in food science technology, preservation, flavors and ingredients, plastics, logistics tech, etc have exploded. Efficiencies that bring the prices way down.
I mean, in like 1982 there were like 6-7 kinds of chips. Lays (four flavors?), Doritos (one flavor), Cheetos, Fritos, probably a local/regional brand. Now look at the chip aisle.
Look at the baked goods...there wasn't 1000 baked goods in plastic shells in your supermarket in 1982.
I'm a big cheerleader for capitalism and the free market, but this issue is 100% the result of the absolutely brilliant effects of capitalism...technology, efficiency, marketing, etc. And it's just about the greatest product line in the world because it's all disposable and people have to buy it again next week.
Nothing has changed in the last 40 years that all of a sudden people will put things that taste great in their mouth. If you dropped today's supermarket/fast food scene down in a city in 1960, with relative prices for the time, that would become the fattest city in America in 5 years.
We have absolutely mastered the efficient, inexpensive delivery of a very basic intrinsic human desire.
And don't dismiss the fact that obesity rates going up all over the world reflects the near global elimination of chronic hunger and food scarcity. People will eat "too much" when they can eat too much.