Because then ugly children will be born, incest, blood mixing...
That is a problem that natural selection can work on and in which natural selection is working on in places where people marry their cousins. This means that the people of Alabama, West Virginia, and Pakistan have a greater chance of making it through some species threatening disaster than everyone else.
Regards,
-Joseph Van Name Ph.D.
Natural selection works by cause and effect. So, Who or What set up cause and effect to act the way it does, since it is extremely complex?