What evidence do you have that it is a cultural shift, rather than, say, a chemical in the environment that wasn't there before, or a difference in the food supply?
Are food options today not more calorie-dense than they were back in time? More easily accessible? Calories have become increasingly cheap and dense to come by. You've lots of processed food being engineered to be highly palatable and calorie dense at an industrial scale.
Plus, cars and all "comforts" eating into your physical activities?
Plus, cars and all "comforts" eating into your physical activities?