What does "by extension" mean? Culture is voluntary. You can (and should) walk away from a flawed culture. Race is an unsheddable fixed attribute.
Meanwhile, both culture and race see more variation within groups than they do between groups, making discrimination a losing proposition even aside from its inherent immorality.
To a limited extent as an individual. One can't simply remake the entire world they were born within.
Btw, the premise underlying this fact is a fallacy. It is the norm for population groups, even meaningfully differing ones, to have "more internal variation" than between groups.
There is more variation in height within sexes than between them, but despite this there is still a very (statistically and culturally) significant difference in average height between men and women.
It's even possible for two distributions to be completely separated by a given variable while still having more internal variance.
Tall women are taller than short men, so if you're hiring a librarian who needs to be able to reach books on a high shelf, you still need to look at the individual. Society as a whole suffers if you run employment ads saying "Women need not apply," so we rightfully deter that.