I don't think its fair to generalize all models as suffering from a lack of self-respect. I'm sure there are loads of confident, outgoing models who are completely happy with themselves.
Just like any other profession though, I'm sure modelling has its fair share of broken people with low self-esteem and a low body image. I'm sure alot of the people they work with are quite hard on them to maintain their ridiculously small size and keep their weight down constantly, and are also constantly harping on them about what is 'beautiful' and what is not. Over time, or if the model was unconfident in the first place, I could see how this would take a huge toll on your self-esteem and self-image, and ultimately their self-respect.