If HR is responsible for managing under-performers in the organization, what happens when the head of HR is the under-performer?

One of the most difficult tasks of a leader is to maintain a team of A players.  Steve Jobs once famously said “A players hire A players and B players hire C players.”  Research has shown that A players outperform B players 2x on essential tasks and 10x on creative tasks.  So why do so many leaders live with mediocre talent on their teams?