I’d guess that leaded gas played a role.

Also, the truth has progressed much further than what many are willing to swallow at once. Not only is our way of life currently potentially leading to an extinction level destabilization of the climate and life cycles that we depend on (which also implies we’re not as important as some religions would lead us to believe), but the Euro-American way of life has been fucking things up for centuries or even millennia in the past. The only way to avoid accepting that our past is full of normalized evil is to bury your head in the sand and say everything is fine.

And add in some racism where they want to blame entire groups for actions of individuals and that line of reasoning says that you must also be to blame for the actions of your ancestors.

And then there’s the human tendency to double down when challenged, which makes it even harder to later change your position, because that doubling down can involve saying or doing things that does directly implicate you in some of that evilness. Or making ridiculous claims to “express certainty” like “if x is true, I’ll do y” where y is something no one would want to do or would even expect someone to do as part of a debate. When you behave like an idiot, changing your mind requires you to accept how much of an idiot you behaved like.

And there’s also group momentum. When the group sees dissenters as deserving hate and violence, it makes it harder to change the group’s direction. Especially if there’s members that don’t really care about the philosophy but are just looking for a “good excuse” to be violent or hateful. It’s not rare to see people treat former allies even worse when they turn compared to those who were never on their side. “Traitor” is one of those labels that can entice a negative reaction like “murderer” or “rapist”, even though it boils down to “person who changed their mind about something they once supported with others”.

So we might, collectively, have too much baggage to stop this train headed for the cliff before we can tell if the track curves to follow the cliff or just goes straight off. And if we do go off that cliff, don’t doubt that, on the way down, those fucking idiots will be loudly complaining that there was no way they could have known and that we should have told them.