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).
