Because our ethics distinguish us in many other aspects of life. Rape, murder and cannibalism are generally frowned upon, we have international laws governing warfare, we're waking up go the destructive nature of our expanding population, we have medical care that generally exists to remediate the sick and wounded, etc etc etc. But slavery and torture are open questions in this millennium, and who would expect us to draw a line for animals where we're reluctant to do so for humans?