The abstract states that it's a puzzle why people are irrationally optimistic. The puzzle is that one would expect this to be corrected through selective pressure, either evolutionarily, or wrt evolved cultural normative expectations.

Let me suggest a rather Neitzschean answer: it's very useful in a society to have the default assumption of the populace be that their prospects are better than they actually are. That's headroom.

If you haven't read him,

Neitzsche suggests that the promise of Christian afterlife, was in effect an extreme form of this: an irrational optimism about the future, which (very usefully) made the actuality of moment to moment, day by day, awfulness bearable, on the premise that it was a transient state.

He also noted that it was useful to associate sacrifice and penury with righteousness, and described the instrumental benefit to the 1% in allowing the masses to take pleasure in the thought that those enjoying a comfortable life in this world, would meet an (per Pascal, infinitely more expensive) bill in the next.

IMO this is "hard to un-see."