Why do objects inside the car appear to be stationary when compared to the objects outside of the car?

Think of the car as a mobile container, holding items being transported, the item doing the transporting is in motion, and attached to the inside are the items being moved. so the total speed of the items being moved is the same as the item/vehicle doing the moving. Because the vehicle in motion is travelling in a different direction to the location being travelled, this gives you the result (the items in the vehicle are stationary relative to the motion of the vehicle in use).