Biological facts
The feel of equilibrium, acceleration, gravity, and so bumps are not feel by the eye but with the inner hear. Which is made of liquid and crystals in circular tunnels.
When the inner hear feels bumps and head motion, it instinctively moves your eyes to soften/filter/remove those bumps.
2 experiences to demonstrate that.
1) when put somebody on a chair, in complete dark, you film his eyes with an infrared cameras, you turn the chair, is eyes will roll too, even if he see nothing
2) this one you can do it yourself, looks at in front of you, at your screen, now shake your head, the vision will not move.
When there is conflicting information from inner hear and eye, you feel strange, it's difficult to focus on the road, and you can have nausea.
What happens in a car
When you are driving, you are looking the road, and all the bumps are filter/corrected by your inner hear, so the viewing of the track is smooth and it's the car cockpit that moving around you.
What happens in a Sim
Because you are sitting and a standing chair/seat, you head is not moving at all, so the inner ear is not working, the eye don't correct the bumps.
That's make 2 issues :
1) you can't focus on the road, the road is shaking and not smooth, it's difficult to drive
2) you can get nausea because you have conflicting information from eyes and inner hear.
How it should be in a Sim
The camera should stayed focus on the track and not locked to car cockpit,that's what my App is doing by filtering bumps and slopes.