My Occam's razor answer is that it's an ego thing. Her ego must be the size of a small city. Not only that - she has an army of insanely overly-paid sycophants providing her with advice and "data" that show she's got a shot at winning. She well knows they have a huge incentive to lie to her. The longer she stays in the race - the more money they make. But denial (in this case self-deception) can be as powerful as love even though nobody writes songs about it.