Because clines are too gradual to place into distinct categories. Race isn't discrete, considering there is gene flow between different populations.
Yes, we do, but like I said humans don't have subspecies because we're
all homo sapiens sapiens, you say it here:
Sapiens sapiens is a subspecies of Homo sapiens. It's a concrete category because there's a significant difference between homo sapiens sapiens and what's argued to be our direct ancestors homo sapiens idaltu. There's not enough variation between different races to consider them to be subspecies.
It's a toss up, really. A small fraction of genetic variation does in fact occur between clusters, that's for sure, but then again if you were to walk from Africa all the way to Siberia, there'd be no set location where dark skin ends and light skin begins. I personally don't consider it a black and white issue, although I personally place more emphasis on clines. Fact still remains that two people from the same race are about as different from each other and two people from other races.
www.sciencemag.org/news/2011/04/what-does-iq-really-measure