Because women really think like this and men really do fall for it. There is always the friend that may be better looking but for some reason wants to steal the boyfriend of the less attractive friend.
The only thing the high school me could ever say is thank you. I was Homecoming Queen which only made my boyfriend more of a target for attraction and I was glad. I left for college and only ever returned to try and help his family get him off the drugs he became dependent on in the years after I left.
Either way it portrays how women are only worth their looks.