Why?

Because you can't define pathology (deviation from a norm) without defining the norm. Modern psychologists dodge this question and implicitly assume various generally accepted moral standards and tenents (a form of circular reasoning) by which they condemn those who don't follow it. It is not science and to make it a science the psychologists should be seeking a rational basis to define ethics. Nobody has done this and until they do psychology will remain a pre-science like alchemy.

In addition, it would also be supremely discriminating if the "Psychopath exam" would, for example, consistently score Asians lower.

That is, a psychological or culturally acquired trait should not be used against an individual without a proof that it actually affects his behavior. You are in essence denying that the person has any agency and it's utterly incapable of learning and acting on society's moral code, which I bet is false for the vast majority of people with sociopathic tendencies.