Because you're looking forward at the platform that's moving down and toward you. Kind of like if you were to stare down at the belt of a treadmill while it's moving - it would be disorienting after a while.
Because even if your claim is correct, and the conflicting visual input is disorienting, the Moon-dweller on a similar contraception could just close their eyes.
Blind people only have 2 out of the three signals so they might be less prone to motion sickness.