Why do people say they "slept like a baby" when babies wake up every 2 hours?
Haha great question. Probably a term of speech that was concocted by people who don't have children or have never cared for them.
Babies supposedly sleep without a "care in the world", hence the expression.
They don't mean it literally?
Because they can sleep whenever so if they're tired they would just go to sleep:P