what prevents A players from hiring B and C players They know it will not end up good for anybody. Also they understand hiring another A player or if possible an A++ player is good for overall health of the company.

prevents B players from hiring A players Ego and in a big corporation, the fear of the hire going ahead of you.

Also another deadly combination is the B player who hires another A player thinking they themselves are A+ player and spoils the fun for everybody.