The obesity problem is only somewhat related to diet.
Primarily, Americans only get a tiny fraction of the exercise most people got in their day to day lives throughout most of human history.
Think of “labor saving devices” as also being “tub of lard transformation devices.”
You make a good point, however most of the “exercise” people get these days help keep the pounds off more by keeping people from stuffing their faces rather than burning calories. Even a serious aerobic workout typically burns just a small amount of calories when compared to an extra hamburger or even a candy bar. A two hundred pound person walking 3 miles at 3 mph typically burns less than the calories in a single candy bar or small bag of chips.