This article is disturbingly enthusiastic about bariatric surgery, an extreme procedure with limited success and serious side-effects.
The long and short of it is that we understand far too little about the human body and what drives weight gain and difficulty losing.
For instance, there are complex health and immune issues (e.g., mast cell disorders) that can drive weight gain and difficulty losing in spite of healthy diets and high levels of activity (and researchers think mast cell disorders alone may affect 10% or more of the population, but was named only 5 years ago and is still poorly understood. What else could be similar that we just don't understand yet?)
Beyond this, what we put in our bodies (various medicines, antibiotics, ingredients in processed foods, various pesticides etc, even artificial light) may have effects on our physiology that we don't yet have a strong understanding of.
Our microbiomes are still a complete mystery to us, deeply impacting how and what we absorb from our food. The same food does not impact every body in the same way, period.
Bariatric surgery is a band-aid over a gaping wound. Until we understand the complexities of human physiology at a more sophisticated level than a high-schooler in physics class (calories in, calories out just doesn't cut it) -- we are going to continue seeing public health impacts in the form of fat and disease -- which may often have underlying causes that the fat is simply the most visible symptom of.
Edit: Seriously guys, is there any other area of science where people are so convinced that anecdotal and small-sample (both in terms of n and longitudinality) observations are valid? There's a huge market for delusion about fat, we're fighting that as much as we're fighting to learn more about the human body.