Even if it is in the owner's manual - and even if it is prominent in the manual - how many of you have ever actually read the manual when you get a new car? I know I have only ever looked when I actively needed something: the meaning of a warning light, how to operate the disc changer, something like that. It is entirely conceivable for someone to own a relatively new car for years without needed to look in there.

edit: and yes, I realize it would probably make sense to check it out when buying a type of car that is relatively new, but that doesn't mean even a significant minority of new owners will.