Hallelujah, someone gets it.
It's not that you cannot get a Caucasian runner that can match a Kenyan, it is that the Kenyan population has a far higher number of people with a genetic predisposition to excellence at middle and long-distance running.
Since most of the world descends from a small group of people that left Africa, the "rest of the world" is more genetically similar to one another than are individual populations descended from those who remained.
We actually see something similar - although more for reasons of commercial distribution - in thoroughbred horse racing. Many countries can come up with a world-class horse, but the depth below that occasional outlier would not compare to the powerhouses of the US (for dirt runners), England, France, Ireland or Australia (for turf sprinters).
So maybe the U.S. at some point might field a 4x1500m (or mile) team that would win a world championship, we'd have no chance against Kenya in a 50x1500m relay.