Blackout, the number of people with the skills and experience to be a director of a public company are limited. Then there is the little matter that Directors are not hired, they are voted by shareholders.
If I am a big superannuation fund, invested in multiple companies, I will have a “stable” of reliable proven directors that I am happy will protect my interests. That is why you see multiple directorships. I say again, Directors are not hired, they are appointed. they are not employees.