This mismatch is what causes the unpleasant feeling. Your brain may not understand modern transport, but it does understand that contradictory signals can be caused by poison, so it reacts as if that's the case. What happens when you're being poisoned? Dizziness, nausea, and possibly vomit. However, the effects are different for different people. Some can handle cars and trains, but not boat rides. Others are fine in the backseat, but they must look out the window. This action can help their brains reconcile that they're indeed moving by taking in external information.
If you've never experienced motion sickness, consider yourself well adapted. The rest of us will be catching up on our favorite podcasts, thank you very much. You can learn more about different versions of motion sickness by watching the videos below.