Because print media uses Photoshop (in the past, airbrushes) to make people look more glamorous than they really are. Check out these before and after pictures. If you click the buttons quickly, you can see boobs grow, butts shrink, and facial lines disappear before your very eyes!
Personally, I think age lines and big butts are endearing signs of personality, but then i'm not trying to sell magazines either. What I find annoying is how many people spend incredible amounts of time, energy, and money trying to live up to impossible (because faked) ideals.