I think they do it for lower age ratings. In video games, people just drop and stop moving, and often there's even no blood. No visible pain or struggle. I think it desensitizes people to violence.

In reality, after a large scale battle the field is littered with people writhing in agony. It commonly takes several hours for a wounded person to die. Several hours of pain, as you slowly realize no one is going to help you and you're not going to make it.