Simple, it’s the human instinct to be alerted in front of threat. Babies constantly learn about the world around them, and to realize which are potential threats. In the baby’s mind, adults only mean larger animals; thus this alert system kicks in. Some babies still afraid of other kids, however most of the time the kids’ mind see those as mangable threats, so the system signals the babies to chill out.
It’s mostly a size thing, and certainly varies from baby to baby. Bigger is scarier. We don’t lose it either, if you saw an animal that you’d never seen before that was the size of an elephant come quite close to you you’d very likely have a pit-of-your-stomach physiological scare feeling as well.
Babies also tend to not be afraid of small dogs and cats, but can often be frightened by really big dogs. My neighbour has a Cane Corso that still sometimes freaks out my 11-month-old.
Latest Answers