Even in a world with monsters stronger than the average human, helmets are still important pieces of armor. Any kind of attack deflection helps, especially given that almost any head damage in a fight can drastically affect the outcome, if not end things immediately. Hell, it'd be worth it just to protect against debris from the inevitable environment-damaging attacks down the line. Shrapnel is just as deadly as any weapon.
Admittedly, it is nice that the author at least tried to give a reason as to why nobody wears a helmet. Not a good reason, but they tried.