Sunny sixteen works because negative film forgives the shadow
People repeat the sunny sixteen rule as if the human eye were a remarkably accurate light meter. It is not. Walk from an open avenue into the shade of an overpass at four in the afternoon and try to guess whether the light dropped by two stops or four; without a meter, I am wrong by a full stop more often than I care to admit.
The reason the rule works in practice has very little to do with our eyes and almost everything to do with the shoulder of negative film. When you err by giving the shadow an extra stop—opening up to f/5.6 when f/8 would have been textbook—color negative simply holds the highlights and hands you back a printable frame. The only real mistake you can make outdoors on a bright afternoon is trusting the sunlit pavement and starving the shade.
My battery died halfway through a Saturday roll last month. For the remaining eighteen frames I set the shutter to 1/250 and only moved the aperture ring between f/11 in open sun and f/4 under trees. Every single frame on the strip was usable, and the four frames where I hesitated and opened an extra half stop were the best on the roll.