Actually, there very much are.
I think what you're trying to say is that not all black people have all the "black people genetic markers," and likewise for other groups. This is true, but this is not necessary to distinguish well-defined groups.
Family resemblance is a philosophical idea made popular by Ludwig Wittgenstein
It argues that things which could be thought to be connected by one essential common feature may in fact be connected by a series of overlapping similarities, where no one feature is common to all.
to quote my own post - well documented taxonomic differences between human races.
Or are you talking about biological differences as they correlate with geographic locality?
"Heaps don't exist because you can't prove an ironclad boundary between heaps and non-heaps."
"Tallness isn't a thing because you don't have a firm, binary definition of when someone is tall."
I am hesitant to recommend it because I have not read much critique of its central points. But I think it's good.
Race is not just a word for different breeding populations of humans. The dilution of its meaning into something so innocuous is ahistorical, in fact it contradicts what was claimed to be the reality of race just decades ago. It's a political concept first and foremost.
No biological theory predicts races. Races are categories rooted in historical economic exploitation and domination that are a posteriori fitted with whatever truthy pseudoscientific justification is catchy at the time. Any categories of humans that biological measurements would come up with would hardly match racial categories. Perhaps the most racialized group that has distinct biological markers are Ashkenazi jews. However, the relation between Ashkenazi jews, Sephardic jews, and ethnic Arabs is very strong.
P.S. A review of the legal history of race in colonial America: https://sph.umd.edu/sites/default/files/files/Harris_Whitene...