Based solely on the part of the definition of the word that means “concerned only with one’s own interests” … if one is going to choose a way to be, then that would be the natural choice.
Reading further in the definition ”… and typically disregarding accepted or appropriate standards in order to achieve them”, that’s also a natural (naïve, unschooled, non “civilized”) mode.
So be cynical! It’s the natural choice. Maybe the real question should be “Why aren’t people more cynical than they already seem to be?”
But that seems to be a non-affirmative question, so I won’t ask that.