The best reason was given by William Gibson (of "Neuromancer" fame. This is the guy that coined the term "cyberspace" if you didn't know this already) when he wrote episode 7ABX13 - "First Person Shooter" in The X-Files.
Mulder: "I've got a birthday coming up." [He models the damaged stun suit] "You have to admit, though, Scully, this is a pretty amazing piece of technology."
Scully: "Yeah, wasted on a stupid game."
Mulder: "Stupid?"
Scully: "Dressing up like high-tech warriors to play a futuristic version of cowboys and Indians? What kind of moron gets his ya-yas out like that?" [Mulder points to himself] "Mulder, what purpose does this game serve except to add to a culture of violence in a country that's already out of control?"
Mulder: "Who says it adds to it?"
Scully: "You think that taking up weapons and creating gratuitous virtual mayhem has any redeeming value whatsoever? I mean, that the testosterone frenzy that it creates stops when the game does?"
Mulder: "Well, that's rather sexist, isn't it? I mean,
maybe the game provides an outlet for certain impulses, that it fills a void in our genetic makeup that the more civilising effects of society failed to provide for."
Scully: "Well, that must be why men feel the great need to blast the cr@p out of stuff."
Well, that about explains it all, doesn't it?
Its all about satisfying the animal impulse in all of us, honed through millions of years of evolution, that has been suppressed and dulled by modern civilazation and currently accepted socio-politico-legal standings. It gives us the thrill of the kill without the kill.