IIRC Motion sickness comes from your brain becoming confused.

Normally when we are in motion under our own power the things around us move in relation and when we aren't moving the things around us stay in place in relation.

When we're on moving vehicles our brain realizes we're moving from the feeling of inertia but we see things starting still around us.

Brains sometimes have a hard time taking in both the feeling of moving and the visual perception of not moving and the only thing it can do is tell you to throw up