Once upon a time someone (or a small group of someones) had an inspirational idea to make a difference. They started small at first, but when they realized the need for donations to make their idea happen, they start a non-profit.
Every non-profit needs a board, but that's not a problem because they already have a team of committed individuals wanting to see the idea flourish.
What's interesting is that at some point in the life-cycle of non profit organizations, the board realizes they can better accomplish their mission by hiring a staff member to do the majority of the work while they provide oversight.
But then eventually the original board members (because of time, finances, family, etc) need to step down off the board. The vacancies are filled with committed individuals, but they weren't there for the original hiring of the staff member. So in a short time span, if the board is eventually all replaced, the staff member knows more about the organization than any board member.
(This is not all non-profits as I'm sure you can have lifetime tenured board members or organizations that never hire staff. But I think it's the majority... nothing wrong with it, just good to know.)