The monsters are like this because GM-Aki (Or was it GM-Ayu? I forget, they were pretty much the same, whatever ?U) made the custom-maps happen and s/he had a very sadistic, "bet'cha gonna have trouble with
THIS!"
...approach to the game in terms of difficulty-level, which made wandering alone in new world lots more difficult--if not severely annoying--than the official version, I assumed it was meant to compensate for how RO-New-World was likely meant with renewal mechanics/overpowered characters in mind.