The Metabolic Winter hypothesis suggests that obesity is partly due to lack of exercise, but mostly due to chronic overnutrition and chronic warmth:
Seven million years of human evolution were dominated by two challenges: food scarcity and cold. “In the last 0.9 inches of our evolutionary mile,” they write, pointing to the fundamental lifestyle changes brought about by refrigeration and modern transportation, “we solved them both.” Other species don’t exhibit nearly as much obesity and chronic disease as we warm, overfed humans and our pets do. “Maybe our problem,” they continue, “is that winter never comes.”
Hmmm. We were plenty warm in the 70s but the obesity epidemic exploded 20 years later.