The heads of very small children are quite heavy relative to the child's size. The concern with a baby is not that they'll hit something, it's that the weight of their head relative to their body will result in spinal injuries.

Consider for instance the $34 million verdict in Texas against Dorel, which failed to warn against placing children under the age of two in forward-facing seats. The kid Cayden was properly restrained in the back seat, but the weight of his head alone was enough to result in spinal injuries that have left him partially paralyzed for life.

We used to laugh at my kid when kid tried to reach for things on the floor and then fell on their head due to the weight of the head relative to the rest of the body -- but that's why rear-facing and five-point is important for children under 2,3 years old.