> shouldn’t they have already made all their mistakes?

Is there some finite limit of mistakes that humans make over their lifetimes? In fact, it would be the opposite - those who are making more decisions are by definition likely to make more wrong decisions, as compared to someone who doesn't make as many decisions.

> Isn’t the board, by definition, paying for people who have a very high chance of making good decisions?

Yes, which is why the salaries for such positions are often so high.