Because it's stomach surgery, not brain surgery.
I have binge eating disorder and emotionally eat. That's not going to go away just because they reduce my stomach or digestive tract. And, in fact, if I don't resolve the eating disorders first, not only will I not qualify for the surgery, I will have all sorts of issues after the surgery and will regain the weight afterward.
When I don't binge eat and don't emotionally eat, I lose weight. Period. I have absolutely no problems losing weight with a reasonable amount of calories and exercise.
The thinking part has to come before the physical part and when the thinking part is working, the physical part comes naturally. Why would I do it artificially?