This is the classic case of "inattentive blindness". There was a book written about this and other "tricks" our minds play titled, "The Invisible Gorilla". Their website: http://www.theinvisiblegorilla.com/
Check out the videos and demos...
When I read this book, I was struck by the realization of why car drivers don't see us. The invisible gorilla test indicates that most car drivers are only looking for other cars or road signs/signals and may indeed not see a motorcycle rider in clear view. I see other riders mostly because I like motorcycles and want to know what they're riding, even when I'm in my car.