I should think this answer is completely self evident, really.
First and foremost their jobs depend on how they look in their clothes. When you are talking about the difference between a size 0 and a size 2, and you are in a paradigm where a size 2 is too large, weight is, essentially, then, life or death.
I don't think models have any self esteem issues that all women every where do not have. Am I pretty enough? Are my boobs big enough? Is my rear end too fat? Is my waist small enough? Are my legs long enough? Is my hair full enough, straight enough, curly enough? Am I feminine enough? The difference for the models, then, is that they are fixated on these physical things on a daily basis because of their professions. The average non-model can at least get so caught up in real life from time to time as to forget or at least forget for a moment some of these physical concerns.
Couple this daily questioning about weight with an almost certainly daily dose of sexual abuse or assault, and you've got a lethal cocktail for body image issues and subsequent development of eating disorders. Plain and simple, men are pigs. Those men who fail to see women as people but rather as objects can commit some of the most atrocious crimes. It's no wonder most beautiful women are empty shells inside. When you can't go to the grocery store without being hit on, can't walk your dog without being whistled at, can't go to a club without being groped, can't get out of your house without being molested to some degree or another, it makes you vulnerable, angry, confused, and just plain stressed out. A stressed mind is not a healthy mind.
We're all of us attracted to beautiful, sexy women. They're pretty to look at. But these women are rewarded solely for their looks. And while everyone who has a job is used by his or her employer to further the employer's goals to one degree or another, these particular people are used as props.
At times they probably want to scream, "I have a mind too," but none would hear. They all know that each minute that goes by is diminishing their over all value because, let's face it, your looks don't last forever. Wrinkles form, boobs sag, skin ages and so on.
So, you've got an industry built on marketing clothes which uses rail thin, ridiculously tall women to make the clothes look good to the average consumer. And it is a cut-throat business, where a designer can make you or break you in a snap. To be the best, to stay on top, particularly when the beauty of youth is fading, one must be positively perfect.
When you hold yourself to an impossibly perfect standard, you're going to fail. Failure, particularly physical failures like a spec of cellulite or a half gram of fat, compounds the self-esteem issues already.
Most of these young ladies had self-esteem issues to begin with, though. It is a rare healthy individual who would willingly submit to this sort of psychological torture day in and day out. You may be tempted to say, "They have fame and fortune how bad can it be?!?" Just remember money can't buy you love, and it **** sure can't buy you self-love.
So they're already a tad fragile, then they're pushed around and treated like commodities until they are too old or too unfit to work, and they're constantly reminded of their situation daily by their mirrors, by the tabloids, by the designers, by the agents, and so on. Honestly, how any of them don't come out with disorders is a miracle.