That is a really good question. I’ve wondered about that too. It may have provided some protection, as it may increase avoidance of future dangerous situations. It could also partially be a side effect of other useful emotional expressions. Or perhaps human behavior grew in complexity (more ways to hurt each other) faster than evolution could improve our emotional processing capabilities. It’s hard to say.

Relatedly, maybe humans had better ways of coping with trauma in the past, through communal bonds, more strongly shared world views, rituals, and so on.