They are insecure and on some level equate men sleeping with them as 'valuing' them as people or liking them. Some women just sleep with guys to feel like they're worth something. Then when the guy doesn't call them back, they get confused and feel 'used' because their expectations of what the sex meant was completely convoluted from the beginning.
There are some women that just like sex, but I'm referring to the type you specifically mentioned.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi