Because those who blame some medium (be it the comic book scare, or the dime novels blamed for Jesse Pomeroy, or Marilyn Manson blamed for Columbine, or video games for whatever other atrocity it's convenient to blame them for) for society's ills never risk their pet bias by testing it against reality. Such people have never been honest and always have an agenda, and I don't trust them to be honest, let alone consider than they know my own mind better than I do.
Wow. Amazing. In one post you attack me for dismissing all people of a particular view out of hand (something I never actually did) and in the very next post you do the very same thing.
Unimpressed.
I'm going to pick one person who is critical of computer games. Dave Grossman. I'm not picking him because I think he's completely right. I've discussed in other threads, in some detail, why I disagree with some of his arguments. But I'd like to explain why I think his views are worthy of being considered and listened to, and not dismissed wholesale as you're attempting to do.
Dave Grossman is actually Lieutenant Colonel Dave Grossman (US Army, retired), and Professor of Military Science at Arkansas State University. During his long and varied professional military career he served primarily in combat roles including the 82nd Airborne and the 75th Ranger Regiment. However he also had academic roles, ultimately as professor of psychology at West Point.
Grossman is one of the few people to actually study the psychology of killing; a body of academia for which he coined the term "killology" (I know, not very original). I thoroughly recommend his book
On Killing.
From his own career experience and from his research, he can probably be regarded as a leading expert on desensitisation. The army, in general, are experts in it as they have deliberately developed sophisticated training regimes with that exact purpose in mind, and the results speak for themselves. During WW2 an estimated 10-15% of combat troops would engage the enemy. By Vietnam, new training techniques had risen that over 90%.
Grossman uses blunt language - he refers to light-gun FPSs as "murder simulators" and I'm inclined to suspect that his work has been exploited by those who just want to ban violent games outright - not something Grossman is aiming for.
More importantly, Grossman doesn't appear to have ulterior motives, and isn't trying to blame society's ills on a particular medium or single aspect. He has used a lot of his research to train law enforcement and military in new techniques for improving the outcome of lethal encounters, for providing coping mechanisms for people who have been exposed to violence, and he speaks widely in civilian circles on the various ways to reduce violence in society and deal with the aftermath of violent events.
As far as I'm aware he doesn't even want to ban violent computer games, but rather wants to better control access by children to all forms of violent media. And indeed, a common defense heard from pro-gamers (and one I am quick to make myself) is "This game isn't intended for children".
Is everything he says right? Are all of his conclusions 100% robust? No. But in general he knows what he's talking about, and he should not be dismissed out of hand.