> external cause of obesity that we haven’t identified yet (junk food is too simple of an explanation, and doesn’t adequately explain everything).

You can't divorce it from the explanation. People get obese from overconsuming calorie-dense rich foods. These products are manufactured to be as savory and addictive as possible, are highly available, and convenient, and very affordable, and most are non-satiating (especially drinks). So yes there are environmental factors, but the reason they matter in the first place is the impact of junk foods. At the turn of the 20th century even relatively sedentary office workers would not gain much weight, because the American industrial machine did not yet create the landscape we have now. No one was buying several bottles of 2L coke, chip bags, jumbo fries and fried chicken, greasy pizza, starbucks lattes, etc.

And to avoid confusion, I want to make clear that avoiding weight-gain at the outset (to the extent of becoming obese), and losing weight, are different. What's sufficient for the former is insufficient for the latter; you do not need to be mindful of energy balance if you are already at a healthy weight and consume whole foods, but if you are trying to lose weight, that will not be enough to ensure a caloric deficit.

A comparison I like is a bag of chips vs a bag of apples. Bags of potato chips average 1200 calories. One would need to consume 16 apples to get the equivalent. I can probably get through an entire bag of chips if I cared to, and more than half a bag absent-mindedly if I was irresponsible. Try consuming more than a single large apple and see how that feels. Fiber is almost as satiating as protein. It may not keep you full as long, but you certainly reach satiety a lot faster than you would with chips (notwithstanding that in fairness they are less addictive without salt and fat).