Mostly because they are invested with the man, and fear the loss, even if he is a cad. Sure does make it hard for the good men out there, eh? Because also cheating men are good liars and the women want to believe them with all their previously invested hearts!!! Tolerance is a good thing then it comes to people, and women have been conditioned to tolerate a great deal of bad behaviors when it comes to men, and it is a little hard to change an entire societies conditioning and the response.