This has been gone over in many other articles, but the short version is that what the player sees in VR is strong enough to trigger an instinctual expectation of motion that, when the body doesn't feel it, causes a nausea reaction. You're seeing something that the brain knows is wrong based on physical feedback; the most likely cause based on data from the last several million years of evolution is some kind of ingested toxin, so systems get purged to remove the poisons from the body as fast as...