(cf. laproscopic bands and gastric bypass being incredibly effective at causing morbidly obese people to lose weight -- they're literally just making the stomach effectively smaller).
They are not that effective. the weight loss is often not enough or the weight returns. Gastric bypass, which is a major operation, will seldom make a very obese person thin and the weight tends to slowly return.
It's all relative, innit? Compared to "telling people to diet and exercise", which is an almost perfectly ineffective intervention (as usually practiced at least), getting morbidly obese people to become "merely overweight" is huge and results in considerable improvements to life expectancy and morbidity.
As summarized by Wikipedia, "median life-expectancy was 9.3 years longer for obese adults with diabetes who received bariatric surgery as compared to routine (non-surgical) care, whereas the life expectancy gain was 5.1 years longer for obese adults without diabetes", citing this meta-analysis: https://pubmed.ncbi.nlm.nih.gov/33965067/
My brother underwent this surgery, and not only did the weight come back, he developed a serious drinking problem (he's in recovery now). Apparently, this a known side effect. All it really did was make his life much harder in the long run. Hunger is more that the physical feeling of fullness.
They are not that effective. the weight loss is often not enough or the weight returns. Gastric bypass, which is a major operation, will seldom make a very obese person thin and the weight tends to slowly return.