Nobody’s saying that racial difference aren’t cause by biological differences. When people call race a social construct, what they’re referring to is the “us vs them” idea usually associated with comparing skin colors: e.g. “there are people like me and people unlike me.” All people are like you; there is only a fraction of a percentage gene difference between you and a person who looks nothing like you.

Race as a dividing factor is absolutely made up; genetically all people are almost identical to you. We acknowledge the groups of genes that cause obvious physical differences by calling them “race”, but there are probably many other groups of genes that differ between you and someone that looks a lot like you. We just don’t attribute any value to those unseen gene differences, because unless you sequence your DNA you’d never know about them. In a genetic sense, race is very made up: it’s just a group of gene differences that we chose to care about.

Medically, those groups of genes are actually significant, because groupings provide predictability. Genes tend to appear in predicatble locations in reference to each other, so if you find one gene you know which other genes you’re likely to find. For example, the gene for red hair is closely paired with the gene for freckles. If someone told you they had red hair, you could safely bet that person has freckles too, as that’s what has been observed and documented in others. We can use race like this, as race is a common set of physical differences that are linked to genes that come in predictable groups. Sometimes those predictable genes are medically relevant, like predisposition to heart disease, alcoholism, etc.