Building on this, there’s also the evolutionary element. Humans continued evolving after civilization, even if the timeframes were too short for much mutation to occur. During early civilizations, and probably most civilizations before the industrial revolution, pessimistic outlooks were extremely common in the real world. As a result, both instinctive and learned pessimism would give an individual more surviving offspring within any given civilization.
Building on this, there’s also the evolutionary element. Humans continued evolving after civilization, even if the timeframes were too short for much mutation to occur. During early civilizations, and probably most civilizations before the industrial revolution, pessimistic outlooks were extremely common in the real world. As a result, both instinctive and learned pessimism would give an individual more surviving offspring within any given civilization.