Why are non-human-animals considered property of humans?

We deny the personhood of animals because we claim that animals have certain “defects,” such as the inability to use language or a supposedly inferior intelligence, that permit us to treat them instrumentally, as means to our ends. But there is simply no such “defect” that is possessed by animals that is not also possessed by some group of human beings. There are, for instance, human beings who are severely impaired and will never engage their environment as actively as a healthy dog. Nevertheless, we would never think of eating such a human, or using her in experiments.