A prospective hire who is an A (in whatever field) wants to work with their kind, and so is less likely to sign up at a company full of B-level people.
Also, as mentioned above, it is harder for many B-level people to recognize and value an A level person in the hiring process. An A may come across as arrogant by describing things as good or bad to a B when they're simply knowledgeable and confident because of that.
And by that mechanism, C players use the "only hire A players" rule to claim that A players are "not good enough", though such objections don't get raised about other C players.
There were some A programmers at Amazon, and they were respected, but they weren't the ones who made the hiring decisions. Since any B or C can veto any hire, A people often didn't get hired in favor of B or C people. (and A people who already worked there, eventually, got excluded from hiring loops because they're "needed elsewhere.")
Also, as mentioned above, it is harder for many B-level people to recognize and value an A level person in the hiring process. An A may come across as arrogant by describing things as good or bad to a B when they're simply knowledgeable and confident because of that.