I have heard this a lot, but personally I have never seen it, so I think it is mostly an urban legend myth. All hot girls I have ever met always had guys asking them out. If fact they usually have the opposite problem of not being able to go out anywhere alone without being hit on.
While it is true that there are a lot of shy guys, there are also a lot of aggressive guys who hit on all the hot women.
So I'm not buying this myth of the hot woman who can't get a date. In reality, what I think is happening is that she doesn't like the guys who are always hitting her up and she wishes some of the shyer guys would ask her out. But I think those guys are more intimidated by the aggressive guys they see asking out the hot girls than the girls themselves. Shy guys need a lot of time to work up the courage, and if they see a girl they like talking to another guy, they will think they've missed their chance and so not approach her.