Because that reasoning – that your own utility is maximized by selfishness – literally cannot be right. If it were right, then it would be the answer all rational beings would arrive at, and if all rational beings arrived at that answer, then none of them would cooperate and everyone would be worse off. If selfish utility maximizing is the correct answer for how to maximize selfish utility, selfish utility is not maximized. Therefore selfishness is the wrong answer.

Considering that in the real world different people will have differing abilities to calculate the consequences of actions/beliefs at various levels of complexity, and differing abilities to deceive others about their own beliefs and actions, there could be a person who perceives himself to be above average in this ability for whom 'hypocrisy' could be the right answer.