I'm asking because I really don't know....
Is there truly anyone for whom diet and exercise won't work and therefore they have to have weight loss surgery in order to lose weight?
The point of weight loss surgery is to physically limit the amount of food one can take in. Food can also be limited just by not eating it. So why is surgery needed?
Clearly there are people who haven't been successful with diet and exercise.
There are a lot of tools for weight loss. Exercise, caloric restriction, psychology, pharmacology. Weight loss surgery is just another tool. Some of us need a few tools, and some need more. IMO, the trick is discovering which tools each individual needs.
Weight loss surgery helps on several fronts:
A feeling of physical fullness. Most people rely on a feedback loop to stop eating. When you've eaten enough, you feel full and satisfied. Some people who are obese have lost (or never had) a sense of fullness. They don't get a feeling that they've had enough to eat - they stop eating when their stomach can no longer expand far enough to eat more. Without a sense of "I've had enough", they have to count calories at every meal - and it's impossible to be accurate in all situations - it's also difficult to know what the "right" number of calories is (more on that later). Weight loss surgery can mean that the feeling of physical fullness kicks in at 3 ounces or a cup instead of 4-6 cups of food.
A hormonal sense of hunger or satisfaction. The science here isn't as clear. Gherelin makes people hungry. People who are or ever have been obese tend to have higher responses to gherelin. No one's really sure if that tendency encourages people to become overweight, or if being overweight causes the altered reactions. Either way, weight loss surgery can discourage the production of gherelin, so you feel less hungry. Gherelin is part of that eating feedback loop.
Decreased absorption - not all WLS has this effect, but it means you can eat more food than you could if you absorbed it all properly. This is a bit of a double-edged sword, because it means that you have decreased absorption of the nutrients you need and the ones you don't. For many people this is a BIG win, because of adaptive thermogenesis. If we take 3 women with identical bodies (height, weight, body fat percentage, activity level) - the one who's always been 150 lb needs 2000 calories to maintain her weight. The one who used to weigh 168 needs 1800 calories to maintain. The one who used to weigh 190lb needs 1500 calories to maintain.