I’m not sure, I know the reason we get motion sickness (car sick, or sea sick, or even sick from a roundabout) is because the motion we see with our eyes doesn’t agree with the motion we feel in our brain, and our brain then thinks this might be because we’ve eaten something bad and so makes us want to throw up. Maybe it’s something like that with the vision, but I don’t know what exactly.
Johns right but its more to do with the motion we see doesn’t match with the motion we feel in our ears. We have tiny hairs in our ears that detect movement and motion. When on a roundabout/car/train etc the motion doesn’t match what we see and we become dizzy.