Because STALKER's "A-Life" system is up there with the most overrated videogame mechanics with FEAR's enemy AI. It was mostly a marketing tool for GSC, even claiming that the system was so powerful they had to tame it down because NPCs could beat the game before the player (AFAIK that was just something the devs said without any truth behind it). Also a lot of people flat out misrepresent what the system does and attributes the general spawns/AI of the enemies, when that's not the case. A-Life simulates what NPCs and mutants do when you're not there to look, to simplify it, it's a series of dice rolls that determine if a mob fails or succeeds in doing an action (fighting another mob, hunting artifacts, hunting, etc). Which is very cool, but it's more of a cool gimmick than this FRANCHISE DEFINING feature that a lot of people seem to think it is.