It's Right Under Your Nose

Why is it that those exceedingly demanding patrons that come stomping up to the Ref Desk wanting your attention because they can't find a book on the shelf (and spend the trip back to the stacks swearing and belittling you and your library) never offer up an apology when the book is discovered right where it was supposed to be according to call number on the shelf?  Why?  Are they too stubborn to be gracious or they just don't want to admit that they were wrong or that they may need glasses?