We discriminate because it's a cornerstone of the human identity. We have a need to believe that we are somehow better than other species in order to legitimize what we see as our birthright to do what we please with the planet and its non-human inhabitants. The need is so strong we'd rather participate in fancy mind-games that help us create these reasons, than to believe that we are simply the most brutal species on the planet. Personally, I'd rather believe that I'm unapologetically brutal. I want to be in touch with that half of me that is so inhumane that makes me perfectly human.