It's because they have no concept of moderation. To them, you're either fat or anorexic. You either eat 500 tons of "awesome food" or 1 ounce of dry celery sticks every day. You sit on your ass all day, or you're working out 24/7. There's no in between for them. They don't realize that anything is possible in moderation, and until they do, they'll always be lardasses.