Hence the word "concentrated". Your snark seems misdirected.

I live in flyover country, and there are no shortage of talented people here, but I wouldn't dispute that the per-capita software developer talent in SF is going to be higher.

I think it's mostly societal perception. For whatever reasons, a lot of people seem to think of professionals in SF and NYC as smarter or more capable than professionals from Boise or Salt Lake City. Just like a lot of people assume those who got into Harvard or Stanford as 18yr olds are smarter or more capable than those with degrees from the large public universities.