The issue is that it can be perfectly healthy to eat around 900,000 calories a year, but if you eat just around 15,000 calorie per year too much, people gain 5 pound a year. That is less than 2% over target, but a weight gain of 5 pound per year, will easily cause significant issues in the long term. 15,000 calories a year is just 41 calories a day or about an half an apple every day.
People do not have to eat significant amounts of food to become fat, even tiny amounts of extra food can easily add up to significant gains. Without a closed regulation loop it is basically impossible to eat just the right amount of food. If people have broken internal regulation loop, they build their own regulation loop and permanently count calories and watch their weight to adjust the amount of calories consumed. Unfortunately there is a lot of noise in weight measurements and a broken internal regulation loop often tries to counteract external regulation. It seems that an unhealthy diet can damage the internal regulation. Gastric bands seem to help because they help to readjust the internal regulation loop and not just make it harder to consume a lot of calories.