I think only the kookiest diets ignore this basic principle. The problem is that most people can't just rationally decide "oh I'll just eat less" or "I'll just eat lower-calorie foods" or "I'll just exercise a lot more and ignore the hunger that comes with it". Well, you can decide that for a variable-length but generally short amount of time, and then billions of years of evolution kick in and hormones make you want to eat more, and eat all kinds of stuff your rational mind tells you is probably a bad idea, but maybe just this once.

Dieting is about tricking that system in one of various ways. Different things work for different people. For some people, no tricks are required but you can't just assume it's that easy for everyone.