Because there's no one else to wag it for him.
or
2 years old.
Cow: "Mooooove over"
Sheep: "Naaaaaaa."
or
Five. One to change the bulb and eleven to say they could do it faster.
or
1,000,001: One to change the bulb and 1,000,000 to rebuild civilization to the point where they need light bulbs again.
or