Society defines racial categories according to made-up rules. The basis of these rules is not biological. All good so far. But, then we have to live with these categories, and they get applied to how we organize society, sometimes explicitly, sometimes less explicitly. And we may wish for these rules to change, or for the ways that they shape the experiences of individuals to change. But then the people who benefit from the status quo have a material interest in having them stay the same. And in fact the people who benefit from the status quo often are in a position to shape society in a way that keeps them the same. You get the idea.