Ignoring the common stance of personality, types and character versus looks and so on– what makes you think the girl is fishing for someone anyway? Why does a girl, any girl, in a club automatically have to be there to hook up? Some people want to get lost in a crowd, meet friends their own age (note: friends), enjoy a social atmosphere whether or not participation occurs, or just forget about life outside of the background music and the drink in their hand.
And why on earth would you think another human being must 'settle' for anything they don't want? If I gave you a dead animal and said, "You won't find better fur anywhere," your first reaction likely won't be, "Oh goodness, you're right! I should take it home and clean it now..." Likely I'd get a strange look, perhaps some negative stilted conversation, and the interaction would end, with you sans dead animal.
There is absolutely nothing irrational about anyone turning down sexual advances that are unwanted, no matter the reason or circumstance. I may dare to venture and say, irrationality is your inability to accept that it can occur.
Simple answer to your question: "Because she really doesn't want anything to do with him" or, "She's not there to get picked up."