Nursery Rhymes

I was just thinking that a lot of Children songs and Nursery Rhymes are kind of depressing and morbid, don't you think? For example...
Hush-a-by baby
On the tree top,
When the wind blows
The cradle will rock.
When the bough breaks,
The cradle will fall,
Down tumbles baby,
Cradle and all.
Tumbling baby from the treetop? @_@

And the one they teach you in Spanish classes about the cockroaches that can't walk because it doesn't have 2 back legs >_<:
La cucaracha, la cucaracha,
ya no puede caminar
porque no tiene, porque le falta
las dos patitas de atrás.

They also have the one about plucking the birds feathers away from head to toes for the kids in French classes.
Alouette, gentille Alouette
Alouette je te plumerai
Alouette, gentille Alouette
Alouette je te plumerai

Japanese also have the one about snipping off the rain doll's head if it happens to rain the following day (Teru Teru Bozu).
てるてるぼうず、てるぼうず
明日天気にしておくれ
それでも曇って泣いてたら
そなたの首をちょんと切るぞ


I'm sure there are a lot more. Why are they so sad and mean? T_T