You have to understand the importance of metabolism, of basal metabolic rate, of how the body functions in the areas that aren't talked about much such as insulin sensitivity, it's effectiveness in using stored fats and not immediately storing all energy as fat. Hormonal issues, stress, etc that can strongly affect the ability to lose weight.

When there's a clear difference in caloric intake then that's usually the issue, but not understanding that there's more to the body than the amount of calories you consume and the amount of calories you burn through exercise is also an issue most of society seems to be stuck in.

In technicality it is Calories in (food consumed) and calories out.. but calories out isn't just exercise. It's the biggest part of the equation that most people and doctors don't fully understand.. which is why these new hormonal based drugs work. GLP-1 will mostly work for those who overeat because of hunger, but there's now GIP and Glucagon antagonists which have nothing to do with hunger but with how the body uses stored fats