Just like anyone else that could be single.. if she is knowingly an actually attractive woman by society's standards... maybe she has very high standards?
Or maybe she has been hurt before... as anyone has been, being an attractive person does not disregard the possibility of being burned from past relationships. Being burned by a previous, toxic relationship can cause MANY people to become afraid to involve themselves with someone else in fear of it happening again, it can shatter some people mentally and emotionally.
Or - maybe she is just focusing on other things right now in her life - such as her career, or finishing up her education... there can be a dozen reasons why someone stays single,
considerably attractive or not.
Not everyone is meant to be in a relationship.. and that can go for an attractive woman.
Some people are meant to stay single, they don't like the feeling of being "tied down" to one person forever ... and that's fine, as long as they aren't hurting anyone else.