Because it's a social construct by definition, stemming from the human desire to label or categorize. Want some somewhat entertaining examples? Take a look at any thread about a mixed *Black* individual and see people struggle to decide whether the individual is mostly, Black, White, etc. The subject of genetics is a very complicated one. Religion though? One can't even fathom all of the flagrant things that have occurred as a result of religion.
We label and categorize because we are different might as well go blind if you think we are all the same . Inside our bodies are alike but we have a outer appearance which I think genes determine also. We don't understand genetics so we play the guessing game with mixed people because their genes are so random.
Nothing wrong with religion its organized religion that is the problem and until someone can come back from the dead and tell everyone what happened or explore the universe not from a telescope or explain the unexplained their is a higher power