We've designed our cities, suburbs, food industry, and society such that the average person is at least overweight, nearing obese. If the average person suffers from what should be an anomaly, perhaps it is less a personal failing than a societal failing and we should try to fix the societal problems generating the poor outcome.
Sugar is in everything. Snacks, restaurant food, drinks, sauces. It puts the metabolism into a bad state and desensitizes the body to its deleterious effects. Before you know it you're overweight, mentally foggy, and pre-diabetic. Not a good place to start working out or going for outdoor activities.
People exercise and walk more when they have energy to do so. Sugar saps that away.
Look at people in other "confined" spaces: submarines, ships, the space station. These people are not overweight for lack of walking.
It's diet.
America is pumped full of sugar.
edit to respond to comments (since I'm flagged / rate limited) :
Someone with a healthy body and mind won't "sit on their ass 23 hours a day". They won't consume too many calories, either.
Sugar pushes the body into more lethargy and higher sugar consumption.