People enjoy shooters just as much as they enjoy lasertag, paintball, etc. It's the thrill of the kill without actually killing anyone.
It allows people to go back into a primative and bloodthirsty mindstate without suffering the consequences. Nothing pumps up the adrelanine for gamers like a nice shooter I say. I mean come on, if you set a Call of Duty fan in front of World of Warcraft and told them they could not get up or play a shooting game until they reached level 50 they'd start foaming out the mouth wanting to bite you!.