Why does reading in the car cause motion sickness?

Motion. Yoir brain is compensating for ye movement of the car as you strain to read so it is like being tossed on a vessel at sea.
Conflictual input. Motion sickness results from your body sensing a discrepancy between what you see (your eyes, focused on the book, say that you're still) and what you feel (your other senses say that you're moving). The discrepancy between what you see and feel causes spatial disorientation, nausea, and vomiting.