I personally think it's because all the images are of people (or celebs) who are "under weight", or have muscles that are "over defined".
You never see a Chubby woman posing in a swim suit, or a man with a bit of a beer belly.
They are always a perfect size 10, flat stomach, pert chest, well tanned, and if male, with well defined muscles.
It's never stated that these models are actually right at the bottom of the scale when it come to BMI.
Perhaps the solution is that all such picture should carry a health warning "in order to get this skinny you need to make yourself ill".
Add in the fact that most cat-walk/magazine models look 14Years old, and 5 stone, it's no surprise to me that normal girls/women over 12 have no chance of looking like that unless they go on a "no food" diet.