Because I don't believe in them. It's always been my theory that if you walk everywhere you'll never have to diet or exercise. And I think the French have proven this over and over again. And that is the real reason French women don't get fat. They're always in constant motion plus they don't put crap in their body.
Having said that, I think this NYC Ballet Workout book has a lot to offer, and not just for weight loss although I'm sure that might be one its effects. Rather, I like this book because it focuses on stretching which like Pilates can really elongate your body, while improving your posture and making you more flexible.