As to the WHY, I’d simply say that pornography makers are a business. They’re creating for a market. For complicated reasons, this is what their market wants. Their market is usually men, often heterosexual men who may be frustrated that they haven’t created a satisfying sexual relationship with a woman. That frustration can be alleviated by fantasies of violence well received by the women. It’s unfortunate, but psychologically satisfying to that particular market.