I have no particular expertise in this area, but I've been making the "we don't know why people are fat" argument to whoever will listen for a while. Doctors (socially, not at an appointment FWIW), academics… I'm actually quite surprised how often they've been thinking along the same lines and/or they are convinced by the general idea.
basically:
People have been getting fatter, to pathological levels for quite a while. The phenomenon is global. The conventional explanation is too much for and not enough exercise, caloric surplus.
This is not nonsensical. We know that caloric restriction leads to weigh loss. We can also see that various other approaches can result in weight loss. Carbohydrate restriction, for example. But, this doesn't give us a complete (or possibly even relevant) answer.
This requires another 'why' and a 'why now.' All it takes is a small sustained caloric surplus over long periods to reach obesity. People have had access to these calories, in large populations or in pockets for a long time. There have been periods of regional caloric wealth and certainly periods of caloric wealth within subcultures/classes. If caloric surplus is the key idea, why do we have these caloric surpluses today.
There were sedentary workers even in ancient times, certainly in the 20th century. Today's sedentary worker is fatter than yesteryear's. Today's carpenter is fatter than a carpenter in 1972. Why? A carpenter in 1972 had plenty of access to calories, similar physical lifestyle.
IMO, this is an epidemic and we need an explanation that works at epidemic levels. We need to explain the change in populations weight, not differences in individual's weight. The answer to the first is probably different to the second.
"Gut biome" may be an answer. Antibiotics. Parasites are another interesting possibility.
In any case, I don't think it's coincidence that fad diet books and other pop science is so successful. We do not have an answer from science. I don't eves think the right question is usually asked.
Why did we all get so fat?