This is purely my theory, entirely unsupported by scientific evidence, but it makes sense to me:

The Cue Conflict Theory is based on the idea that you are seeing one kind of motion with your eyes, while sensing a different kind of motion in your inner ear, and somehow this causes nausea (for me it's mostly headache at first, but if I let it go untreated too long it can become nausea).

It's my belief that being in *control* of the motion (driving the car, piloting the plane) is an additional layer of insulation against the cue conflict messages. Somehow, being in control of the thing that's causing the conflicts tells your brain, for example, "it's OK, I know that the inner ear feels like I'm rolling sideways when my eyes see a yaw motion. I told the plane to do that, don't worry", dampening the cue conflict effect.

This could be an explanation why some people get carsick even in the front passenger seat, even when watching the horizon, but don't get carsick if they're driving. Or a pilot who's fine if he's at the controls, but gets sick if he's a passenger.

I think that translates into the video game world as well. This is something that I very specifically encountered during LAN parties. I always got motion sick very quickly when watching a friend play, but was fine when I was playing the very same game myself. I think that being the one making the control inputs gives my brain an additional "cue". I'm the one requesting to my brain that I want to turn left, the brain is managing my muscle controls to operate the mouse or joystick, and so my brain is compensating and deliberately ignoring the cue conflicts from my innner ear.

Anyway, that's my theory.