Have you ever been on a plane that suddenly hits rough turbulence or drops a few hundred feet? I have, and you often have no warning. Anything and anyone not strapped down go flying.
The only way you could be sure to keep your baby safe from a sudden jolt of the cabin is to keep a tight grip on them for every second of the flight. Do parents do that? Not that I've seen. The keep a grip on the baby for takeoff and landing, or during patches of turbulence, but the rest of the flight the baby is not being actively gripped by the parent.
As far as I know, no baby has ever smashed their head against the ceiling of a plane due to a sudden drop. However, it is going to happen one of these days, and I feel terrible for the parent who has a dead or disabled baby and is kicking themselves for being to cheap to by the baby a seat.