While a videogame might be a 3D world, both of your eyes see the same image on screen. So it may seem to be in stereo but you are actually looking at it in mono. You can easily judge distances between objects that are equally near, but not if they are moving toward or away from you.
Depth is less of an issue at distance, where the difference between stereo and mono is negligible. First person shooters work really well because enemies are often far away. The only accuracy that’s required is effectively two dimensional (place your reticule on target, then fire). However when enemies move in close in a first person shooter, the player becomes acutely aware of how little information he actually has.
Distance to action is therefore an important rule for good gamatography (game camera design).