Because all girls love attractive bad boys. And nice guys never finish first and they will always be at the bottom of the love chain as they await their fate of being friend-zoned.
There are countless examples of boys behaving like dicks to their girlfriend(s) and still receive the respect no matter how abusive and inconsiderate they are. Jot this down in your mind; the nastier you are the better. It means that you will have a better love and sex life than someone (the nice guys) who do not heed our advice. Take our dear Edward Cullen from Twilight for example; he's so bad that he invades the privacy of the girl of his life without her permission, yet received the love and appeal from Bella and a huge number of fangirls all over the world. Isn't that totally romantic and sexy?
Being nice is like throwing yourself a death penalty in your perfect life or shooting yourself in the foot. Long story short; don't be nice, it's not worth your time.