Because he's sitting on top of both, which means he needs to have the confidence of his shareholders. Shareholders will push him out quickly if they think he is

Because he's sitting on top of both, which means he needs to have the confidence of his shareholders. Shareholders will push him out quickly if they think he is incapable, more so at Square than Twitter. Frankly I am surprised they haven't already, but I think the fact he keeps a lower profile with Square it has helped his longevity.