Because, it works, and if they admit that, they have to admit that they have been lying to us all along.
A natural diet consisting of mostly meats (with their attached fats) and vegetables is the way humans have eaten forever. If breads and sugars were natural foods, the metabolism would have no problem handling them. But as it is, a shot of sugar/flour spikes insulin (this includes today's luscious, hyper-fructose fruits), which in turn drives down blood sugar and causes lethargy and confusion quickly.
Meat causes little reaction from the pancreas, and fat causes virtually NO metabolic reaction from insulin. The body is composed of muscle and saturated fat. When it asks for these nutrients for repairs and growth, and I feed it sugar and grass, it can do nothing with these things but eliminate the fibrous one and store the other as fat, then ask for protein and saturated fats again. THIS is why the Atkins diet and others like it work...because when you give your body what it REQUIRES, it is not necessary to keep asking for these needs to be met continually, therefore, the appetite is suppressed in the proper way.
My middle daughter (now 8) was having a lot of trouble in kindergarden and 1st grade, to the point that the school wanted us to give her Ritalin. Around the middle of the day, she would start acting up. Any stress or annoyance would put her into hysterics. I'm talking about uncontrollable hysterical crying fits lasting 20 minutes or more, until she collapsed into a nap.
I took her out of school and homeschooled her (we were doing homeschooling with the eldest), and I started doing some research. Turns out she's hypoglycemic -- her blood-sugar level, unless managed, would drift too low for proper brain functioning. She was getting a morning breakfast which was high-carb (pancakes, frozen waffles, corn flakes, whatever). Food made from flour and sugar is digested quickly, sending the blood-sugar soaring, followed by a crash a couple of hours later
I switched her to a high-protein/fat breakfast (chicken patty with melted cheese, microwave some frozen meatballs, etc). The protein and fat are digested slowly, keeping her blood-sugar level over a longer period. Now she's perfectly fine. We still have to make sure she gets a good lunch, and a snack around 3-4 to tide her over til dinner, but no more hysterics and acting up (or at least not more acting up than is usual for a girl her age)