A number of reasons... the motion is constant... there is no acceleration or deceleration. Were moving at the same velocity constantly. Additionally, at the equator, the Earth is also spinning at nearly 1,000 mile per hour on its axis. All of these motions are constant and do not impose the sense of motion of the semi-circular canals of the inner ear's vestibular system, since that system require accleration to move the fluids against the tiny "hairs" feeding our brain for a feeling of motion...