Observation: When you stick your head out of a car window at speed, you're going from almost no wind to a lot of it very quickly, and that's bound to take your senses by surprise, regardless of whether you're consciously expecting it or not.

Helmet-less motorcycle riders - inadvisable as their choice may be - have the advantage of being able to ease into it from standing to moving at whatever their cruising speed is over the course of a reasonable acceleration.

... though I note that other posters have given reasons why it's still not entirely pleasant for the motorcyclist.