Why does VR make people feel sick?
Because VR produces the reverse of ordinary motion sickness: your eyes report movement while your inner ear reports none. It is the same sensory conflict mechanism, running in the opposite direction.
The core conflict. In a car, the vestibular system detects motion your eyes do not see. In VR, the headset shows you moving through a world while your vestibular system correctly reports that you are sitting still. The brain receives contradictory information about whether your body is moving, and — on the leading hypothesis — interprets sensory conflict as possible poisoning, triggering nausea.
The VR-specific factors that make it worse:
Artificial locomotion. Moving with a thumbstick while physically stationary is the single largest trigger. Teleportation movement exists specifically to avoid it, by removing continuous visual motion.
Latency. Any delay between head movement and the display updating produces a mismatch the brain is extremely sensitive to. This is why motion-to-photon latency is the critical hardware specification, and why early VR was so much worse.
Low frame rate and dropped frames, which are far more consequential in VR than on a screen — 90Hz is generally treated as a floor, and stutter causes immediate discomfort.
Incorrect IPD. If the headset's lens separation does not match your interpupillary distance, the image geometry is wrong and eye strain and nausea follow. Measuring and setting it correctly is worth doing and frequently skipped.
Acceleration in particular. Constant velocity is tolerated far better than acceleration, deceleration and — worst of all — artificial rotation, which is why snap turning exists.
Camera movement not initiated by you, such as cutscenes or being moved by the game.
A field of view mismatch, and looking at things very close up.
What helps: teleport and snap turning, a static reference frame such as a cockpit or a virtual nose, short sessions, a fan providing airflow, and habituation — susceptibility genuinely reduces with repeated gradual exposure.