My own observations. Sedentary lifestyle does not excuse the US for being so high compared to others. Look at the rates of South Korea and Japan. They also work in offices, transit or commute daily on trains or buses, elevators, escalators etc. It is a combination of things.
For example, Italy has a stereotypical perception of fatty pastas and pizzas. But most Italians probably home cook their meals. Fresh pasta sauce crafted from fresh ingredients free of artificial flavors, preservatives, sugars. The difference between a can of Chef Boyardee Ravioli and homemade pasta is big.
I also think inventions like the Microwave, the 'hoarding' of bulk and big size foods has also contributed. How many people would wait for parents to come home to cook food? Now how many make themselves 2 hot pockets as soon as they are home. Couple that with the crazy adoption of soda. What was once exclusive to restaurants, diners, shops and machines, is now readily available and consumed with every meal. How many people now have several glasses or cans of pop a day.
Finally, America has seen a drop in team sports participation. Less kids are joining football, baseball, basketball leagues. Less kids are actually playing outside or getting adequate exercise. Their lifestyle consists of being driven to and from school, where they come home, watch tv, eat oreos, doritos, junk food en masse. But this is again anecdotal generalization. Just crazy to me. Obesity needs to be looked at soon just like live cirrhosis is to alcoholism or lung cancer to smokers. Instead, we see things glamorizing obesity. The word fat is no longer used anywhere. It is plus size, curvy, voluptuous. Things like body positivity and fat acceptance I feel are even normalizing. Look how many are saying 5'10" and 210 or 215 pounds isn't so bad. However, Americans are easily eating 600-800 more calories per day now than they were in 1970. Portions are bigger, things like chicken or beef now consumed 2 or 3 times a day. BMI is still a very observable and easy to use method to gauge obesity on average. Because the average American isn't 6'5" or muscular like The Rock. It boggles my mind they are predicting in just 13 years, we will see over 50% of all American adults be considered obese. Not overweight but obese. Now imagine how many will move onto being severely or morbidly obese.