Why do people say they “slept like a baby” when babies often wake up like every two hours?

Babies wake up frequently because they need to be fed more often than adults.

Babies are also really good at sleeping through things it would be difficult for adults to sleep though.   They often just fall asleep without regard for social context, location or stimulus.  

Do you fall asleep while you're in a grocery store or dentist's office or every time you get in the car?    Yeah, didn't think so.