This is a unique time in history for having excess food calories readily available. Fewer people (farmers) than ever before are producing more food calories—every farmer in the U.S., for example, feeds 150 million people. And, people can obtain a full day’s worth of calories (2,000 to 3,000 calories) for an hour’s worth of labor at minimum wage. Foods are also engineered not only for higher calorie content, but to leverage our natural cravings for sugar and fat, which are much more rare in the “natural” world.
Part of this trend is the result of a nationwide effort to solve the problem of hunger, which was prevalent (especially among rural poor) as recently as 50 years ago. Part of it is just the inertia of agribusiness, which, for example, has precipitated the subsidizing of corn because it is one of the most calorie dense foods on the planet.
I would also argue that urban sprawl and automobile-centric planning is a culprit. That is quite obvious if you compare people who inhabit a pedestrian friendly city with people who live in that city’s surrounding suburbs.
Cows aren’t the greatest example, since they are mainly bred to gain weight, but a cow raised on grass generally has to walk around to get the grass (which it is designed to digest). A cow that is fed (high calorie) corn is generally kept in a pen and fed to deliberately fatten it. Cows don’t naturally eat corn, so they get sick and require antibiotics while they are being fattened.