Why just why?

They say if your man is cheating you would want to know right... ? Ok so what happens when the other woman tells you your man is cheating? Sending you the evidence and all. Why do women still stay. People are always saying “His gf deserves to know!! Etc etc”. Ok so when you tell her she does nothing and stays ! Actually starts to pick a fight with you instead of the cheater. I just don’t understand women at times.