By my reading of Munger, it’s not necessarily that people in general are optimistic, but rather that foolish optimism is one of the flavors of human misjudgment. Ever the practical ‘psychologist’, I don’t believe Munger focuses on diagnosing the causes of the various forms of human misjudgments (beyond their categorization into these forms) so much as he does on prescribing their potential resolutions, or ‘antidotes’ as he says here.

The math he refers to is simply high school level probability math.