Economists have many hypotheses to describe how consumers consume. Some suggest we consume based primarily on our current income. Others say we also care greatly about keeping up with the Joneses. Still others allege we decide how to spend money by considering what our financial situation will be from here to eternity. A key feature of the last one, dubbed the permanent income hypothesis, is that people smooth their consumption. In times of plenty, they will save and use this money and/or debt during times of want. In this way, they can maintain a constant and (for most) gradually improving lifestyle. This hypothesis may or may not be the best one for describing consumption generally, but for my money it's spot-on in describing how people diet.
I see people as having a set amount of calories they consume on net. Because of smoothed consumption, all dieting does for most is increase volatility without changing that net number. We hear dieters describe this all the time in decidedly moralistic terms: "Yes I can eat that cheesecake because I was good yesterday" or "I was naughty at dinner, so I'd better run a few extra laps tomorrow." The fact that Activity X burned 1,000 calories doesn't mean my net calorie intake goes down for the week, it just means now I can treat myself to dessert! There's nothing wrong with this, but it won't result in a reduced figure (mmmm, puns).
So if weight loss is the goal, why the self-defeating smoothing? My preferred explanation tastes of Hanson: dieting and exercise is ostensibly about fitness, but it's really about signaling and self-deception. Going to the gym or using fat-free dressing communicates something about ourselves both to us and other people, and we enjoy this narrative too much to let truth get in the way.
My homemade chocolate amaretto cheesecake gets in the way sometimes, too.