This is a question one often hears asked by libertarians. It exhibits a confusion. It is like asking, "Why should we expect someone to suddenly drop their self interest when he is appointed coach of a basketball team?"
In the Misesian sense of self interest, we do not expect them to, because it is not possible. No one ever drops their Misesian self interest, because for Mises, "self interest" simply means "what ever one's self is interested in." And Mises is (rightly) contemptuous of the view that this means that everyone is only interested in personal gain in a narrow, materialistic sense. For Mises, St. Francis was acting in his self interest in trying to aid the poor in the sense that doing so was what interested him, just like Warren Buffett was acting in his self interest in trying to become very wealthy, as that was what interested him.
In the narrower sense in which "self interest" is often used, as meaning exclusive concern with personal, material gain, there is no reason to think that a politician is any more moved by that impulse than is a bricklayer, or a doctor, or a basketball coach. A bricklayer is of course interested in making money by his trade, but if he is a decent human being, he hopes to do so by building his customers solid structures that will serve them well. A doctor wants to be rewarded for his services, but if he is a decent human being, he hopes to do so by healing his patients, not by selling them quack medicines.
What differentiates the politician from these other trades is not some delusion that he is some superior sort of human being, but that he has a different object as the focus of his trade. Rather than building solid walls, or healing the sick, his attention is (or ought to be) devoted to custodianship of the general arrangements that make social life possible, and as good as possible. His professional concern is not with how to build a brick wall that will support the weight it must carry, or how to heal someone suffering from pneumonia, but questions such as "Ought we to have intellectual property laws?" or "Should we permit fractional-reserve banking?" (And note: the need to address such questions does not disappear in ancapistan.)
Once this role is recognized, there is no basis for suggesting that decent politicians must be regarded as "magical" beings with powers denied to ordinary mortals, or that they have to be less self interested than everyone else. All we should want of them is that they should be decent human beings, of completely average self interestedness, with a talent for attending to general social arrangements. And if some anarcho-capitalist should wish to deny that such beings exist, they might consider that, for instance, Murray Rothbard put himself forward as someone who had sacrificed his narrow self interest (in terms of academic advancement) precisely because he had such a talent for determining optimal social arrangements.