[AlotOfReading]

One of the major issues with biological race realism is that common race classifications do not form monophyletic groupings. If you already have a set of definitions you need to fit, any sufficiently large set of features can trivially classify those categories. It doesn't matter if the groups are race or "people who like licorice". When the features and population sizes aren't uniformly distributed, you can even get very strong correlations, without there being a meaningful underlying basis.

The social definition is used because that's a most scientifically meaningful and useful definition that avoids many of the issues with biological race realism.