In the past they were spread around or sometimes in research (Bell), now they're clustered in the big 4 (Google, Apple, Microsoft, FB) and SF.
The issue is that people are mostly context. Some programmers do well at Google and wouldn't do well in another corporate environment, and vice versa. "10x" is real-- hell, sometimes it's 100x or 1000x-- but it ignores the fact that it's not always the same people who are 10x. Drop an R&D innovator into a subordinate role where he's just expected to "get shit done, fast" and churn tickets, and you'll likely have a bored 0.3x or toxic -5x programmer.
10x is shorthand for "as important to the business as an executive", and good engineers often are more critical than many highly paid executives. The problem is that the terminology is a bit negative on "average" programmers and the idea that "10x" is a permanent identity is a bit silly.