Ladies why are women like this?

Girls always tell guys to know what they are getting into and need to make sure he knows he's with the right girl before he gets in a relationship. But I know many girls who break up with their bfs in 2 weeks because they felt like they just rushed into a relationship. Why is it ok for women to just heartbreak guys like this? If a guy did this the girl would be devastated and would probably be revengeful