Because "realistic violence" is not really "violent" at all. A bullet doesn't hit a person in real life and a fountain of blood pours out. These kinds of effects have to be exaggerated in every form of visual entertainment as well, including movies and TV shows for it to even be visible. There wasn't anything particularly "unrealistic" about the violence in Spec Ops any more than other games. The subdued elements actually made it more disturbing.
Disturbing doesn't mean more violent. It's why many of the most successful horror films have very little violence, if any at all. It's about building the atmosphere, the circumstances the characters find themselves in, the plot, etc., all of which the game handled just fine.
If you're looking at the civilian WP scene and the only thing you can think of is "LOOK AT THEM COLORS. SO CARTOONY. WHERE THE BLOOD?", you've missed the point entirely.