Everyone, or at least almost everyone, acts according to personal interests. There's a whole branch of political science, Public Choice Theory, that deals with this. Where did you get this idea that altruism was common?
How did you get the idea they think anyone is being altruistic? Usually the complaint that people won’t vote in rational self interest is suggesting that people are voting based on irrational evaluations of their self interest: for the benefit of their unlikely future selves.
Public choice theory explains why irrational collective results obtain from individual rational behavior. To get rational collective results requires that people act altruistically, favoring the collective over themselves.
And they are asking why it is so hard to get rational collective results, aka in your wording why is it so hard to get people to act altruistically. So the answer to your question “why do you think altruism is common* is still “what are you talking about”.
Altruism isn't common because we're created by evolution, and nothing is created by evolution without a selfish (for the genes) payoff. Supposed altruistic behavior in evolved creatures always has a selfish alterior motive (group selection, tit-for-tat payoff, for example).
Public Choice theory is "a whole branch of political science" the same way "historic materialism" is though, with Buchanan instead of Marx, as it was created with the same kind of ideological motivations, with “state bad” instead of “capitalism bad” as the alpha and omega of the discipline. Interestingly enough, both shared the same contempt of democracy.
