Because Puritanical Americans would rather eat ground glass than talk about sex with their kids.

I actually heard a theory that in old Europe- when the mother was going to deliver a baby, the man was designated to "boil water" [superstition]. This in turn would release heat from the fireplace [if done at night] and storks would usually gravitate toward the chimney for the warmth. This was important because in most culture, Storks were seen as being a good omen of God, sending his protection/refuge/mercy. In fact, some languages [like Hebrew] the word for stork literally means those things.
I'm not sure this is the root, but it sure was a cool story to hear!