Because all their role models since childhood have told them that they need to get in touch with their inner bitch if they want to get anywhere in life; that it's fun, interesting, and exciting to be a acid-tongued harpy; and by the time they learn that this technique only works for a very small minority of women who don't mind living alone, have no real friends, and for the most part only exist in movies or on TV, it's too late for them to learn self-control -- which in turn makes them only
more lonely, bitter, and unpleasant.
Just a theory.