Most likely: Playing the field - she can be highly sexualized/masculine ("slutty"), and she can just fool around with different guys for sexual gratification or for free dinners/gifts. She knows she has power over men, just due to her physical appearance. As a girl remarked to me today "I can get a man to do just about anything for me"
Less likely: Simply enjoying her freedom (God bless America). She just doesn't want to be tied down with a boyfriend. Can be related to high sexuality.
Even less likely: Emotional problems - Depression, anti-social personality, severe trust issues, severe self-esteem issues.
This tendency may be a good or bad thing, depending on if you're trying to hook up with her or if you're trying to handcuff her into becoming your GF. Also, blame chump males for giving her such a sense of entitlement as an attractive woman.