Maybe she is sorry things ended with you. Maybe you are comfortable to be around and she knows what to expect with you. Maybe she wants you back. Maybe she is insecure and is afraid there will never be another special person in her life. Maybe she just enjoys being with you as a friend and she enjoys not having the pressure of a committed relationship. There are any number of reasons. Why not simply ask her? Perhaps that is one of the reasons you split in the first place...lack of communication(?).
