Why do women say that being "nice" will pay off someday when you get a wife?

They say that the "bad boy" gets a lot of girls because the girls are immature, and when they get older they will marry a nice guy. Do they really think that marrying a woman who has had with every sbag in town during her "young and smoking hot" stage but now wants a "good man" since she is 35 (and her looks are deteriorating so she needs to land somebody who can support her) gives a guy an incentive to be nice? If anything it is saying "be nice and after we are done rewarding all the ******* out there for being bad, we will give you all the responsibility of taking care of us and being monogamous.