Volunteer position.

No time?

Vacate the seat.

No new takers?

Then pay people to take the seat!

It is possible for a charity to keep administration costs so low, that their beaurocracy becomes inefficient.

And we should also be aware of the fact that new entries to charity boards can bring fresh eyes to the goings on of the board, so it can have a quality control or audit effect.

Which is why they don't do it.