You're up against hindsight bias here, PG. (http://www.overcomingbias.com/2007/08/hindsight-deval.html) People don't realize how absurd the future looks when you have to predict it in advance. (http://www.overcomingbias.com/2007/09/stranger-than-h.html) 500 BCE seems much stranger than 2008 CE, which seems very normal by comparison - so people look back and see a steady progression toward normality, things getting less absurd over time, and they expect this trend to continue. (http://www.overcomingbias.com/2007/09/why-is-the-futu.html)

People don't realize how counterintuitive moral changes look when you have no advance idea of where you're heading. (http://www.overcomingbias.com/2007/03/archimedess_chr.html) So they don't use the kind of cognitive strategies that would have been necessary for, say, Archimedes of Syracuse to question slavery. (http://www.overcomingbias.com/2007/03/chronophone_mot.html)