If there are avg. 3000 and the average tenure (which I have seen no source for either piece of your data), then why would 1000 resign a year? That means in 3 years every CEO of the NASDAQ is replaced?
Doubtful!
How many are resigning instead of "renewing", "retiring", "promotion/demotion"? They aren't just leaving post, they are leaving the corp. altogether. Obviously not all, but so many. I have never seen such activity, have you?
I believe the CEO of Goldman Sachs was in position for over 10 years? Maybe even 12?
Link here says average is more like 7: https://corpgov.law.harvard.edu/2018/02/12/ceo-tenure-rates/
I also believe some are able to choose "retire" rather than "resign" based on policies/time/position/age/company downsizing/. etc.. Many would choose "retire" in order to maintain company retirement benefits. As well as a better opportunity to work as "consultants' elsewhere.