How do you know? Maybe no one's ever tried to blow themselves up?
Maybe they've never delt with someone like Royce...I mean, Humans want to live...right? Why would we go kamikazi on ourselfs?
True they may never have had someone do that ..but i mean if i ended up on that planet myself i would know i'm screwed ..sure i'd probably survive for a while ..especially if i were up against these 3 preds ..but in the end i'd know i'd probably die ..or if not die ..never leave that planet...but thats just me.
Also that guy had been stabbed trough the stomach and hoisted up ..i'm sure he valued his life and wanted to live up until that point ..but he didn't die by his own hand the grenades were for the predator ...the guy was already screwed.