My immediate thought is that this is due to visibility. You can churn through the lowest level positions for a while before questions are asked, but you can't do that for CEOs. Its also the case for other prominent positions. If someone with a big following joins and then leaves very quickly someone will comment on it and if its big enough an article will be written. No company wants to be noticed for people leaving, but they'll make a big song and dance when big wig from X company moves to them.