Choirs is another example of the Ouija Board effect: broad noisy but authentic input combining to high quality signal.

With enough people, choirs always sound great, even if none of the singers are great.

Nearly every singer has imperfections, but randomly distributed around the true note.

The average of all of that is the middle point, which is on key.

Also, everyone is authentically trying to sing to the best of their ability, and singers who are less accurate likely sing quietly, while more confident singers will sing louder.