arguably he grew up as one of the more sheltered members of the crew, alongside... usopp probably also rather sheltered, possibly moreso? he didnt kill his parental figure after all. everyone else lived either with their life on the line or always fighting. and also some members inherent fear lol

honestly all of coward trio are like that: they scare easily because they arent used to confrontation, to fighting. usopp honestly has less of an excuse since he's the sniper and also his timeskip location was kinda dangerous, whereas chopper and nami were *studying* how to better navigate/doctor