Beauty is in the eye of the beholder!
If she really is "good looking" she need only "flirt (back)" when she is interested in a man who flirts with her.
There are basically 3 things that determine if a man will ask a woman out.
1. Physical attraction. Guys don't want to date women (they) find unattractive.
2. She's personable, easy to talk with, and has a similar sense of humor.
3. She has the ability to subtly flirt to let him know she is attracted
Sometimes a guy will see an attractive woman and assume she already has a mate/boyfriend or significant other. Men have never enjoyed being rejected and in this day and age they also don't want to come off harassing anyone either.
A woman who acts (aloof or uninterested) believing that will cause men to "work" to get her attention is likely to spend a lot of time alone.
Neither men or women "enjoy the chase". They want to meet in the middle these days where they acknowledge a (mutual) interest.