Their income depends on how they look, and don't forget that they have LARGE incomes! They can afford to hire personal trainers and personal cooks!
They are driven by the media, who photograph them and comment on how fat or thin they look. This has been going on for decades, way back to when we admired the skinny (but ever classy) Audrey Hepburn. It's so easy to get caught up in that, but it's not worth it if you have to devote hours a day to exercise and diet to look like they do. Why do we have to get down to a size 2? What's wrong with a 6 or an 8?
It's worth it to work hard to get in shape, but it's not worth getting depressed if you're not skinny. Read, read, read, and be smart with your workouts and your eating.
For what it's worth, take it or leave it.