The “sudden” change is easy to see in the supermarket. Most people don’t shop at Whole Foods, Sprouts, or Central market. Go to Walmart and admire all of the densely caloric colorful boxed foods.
Did you know young people actually use door dash/uber to get their fast food? It’s wild. So not even expending a few calories to get it but also wasting a bunch of cash.
The drive to eat and what is strongly influenced by gut biome. Fecal transplants have created new food preferences.
Even saying diet and exercise are the proximate cause may be an oversimplication. We know that the same level of activity and food intake can produce different results in different people. That doesn't violate thermodynamics. There are caloric expenditures that are not measured in such studies, like incomplete digestion and thermal regulation.
People shouldn't eat what they are eating, and they should have alternatives that are healthy for them (which also varies by person, but is almost never equal parts transfat and fructose with a petroleum based dye to make it pretty and to cut the sweetness, enough salt to cure a whole pig).
diet and exercise is clearly the proximate cause of obesity, but the question is what drives diet and exercise, and what changed so suddenly?