Why are there references to death in children's songs?

Why are there so many references to death is children’s songs / nursery rhymes?

Take “It’s Raining, It’s Pouring”-
It’s raining; it’s pouring.
The old man is snoring.
He went to bed and bumped his head,
And couldn’t get up in the morning. (obvious reference to death)

Also, “Ring Around the Rosie,” which is sometimes interpreted as a description of the black plague.