I see quite a bit of anti-science / anti-fact / irrational stuff on all political sides.

For the vast majority of people beliefs are not about being right but about belonging to a group. We believe to belong. When a part of our belief system is threatened, even by fact, we perceive that as a threat to the integrity of our group and fight that threat as if it were an invading army.

I say "we" because I see zero evidence that any group of humans is above this. Take HN for example. Certain things can't be discussed here, and I'm not just talking about the usual SJW/anti-SJW/whatever or race/IQ flame bait. Try bringing up deep critiques of Silicon Valley culture or business models, or anything that might be superficially classified as "woo" or any non-dismissive discussion of the culture that surrounds it. Sites based on the Reddit model of upvote/downvote are popular because they give us an easy way to LARP our intellectual herding behaviors online.

Individually humans are quite weak, but in even small groups we become dramatically stronger. We're a highly social species that evolved to prioritize group membership over most other things because it's critical to survival. Historically being right but alone could be a death sentence while a socially popular complete loon would be fine. This is still true to a great extent. People who believe utter lunacy but are popular in some group are often healthier, happier, and wealthier.

For most of human history intellectual herding behavior didn't have much impact because most of what we believed was legend and myth, not fact, and fact and logic had little bearing on our existence beyond immediate activities like farming, hunting, etc. We evolved in a setting of small tribal groups that lived off the land and the most important thing that helped us survive was group membership. Legend and myth are not devoid of information by any means. They encode moral and ethical ideas, social norms, and psychological coping mechanisms. The factual claims they make are wrong (if taken literally) but they're claims about things that ancient people had no real knowledge of anyway (creation, things long ago, the cosmos, etc.).

Unfortunately as we went down the path of science and industry we went down a path where fact, reason, and logic are increasingly critical to our survival even as they pertain to things that are very far from our individual experience. In this mode of existence our tendency to prioritize group membership over these things might be our biggest weakness. Things like "is accumulating atmospheric CO2 dangerous?" or "are GMOs dangerous?" have become cultural signifiers and thus indicators of group membership. Believing the wrong thing in a given group can lead to ostracism.

If we do experience some kind of global tragedy of the commons I will cite this as one of two primary causes. The other primary cause (mentioned elsewhere in this thread) is game theory and the difficulty of coordinating all-cooperate responses to wide scale problems where defection has potentially high short term value.

Even more unfortunately I see it getting worse as we deploy science (psychology, machine learning) to learn to exploit these tendencies to manipulate one another as part of a larger global war between nations, corporations, and pressure groups.

To criticize Silicon Valley in a way that's sometimes unpopular here -- I'd say anyone working in social media today is guilty of building systems deliberately designed to weaponize these tendencies against their fellow human beings. We're building systems designed to make ourselves collectively more irrational because it makes people click ads (or support political causes and candidates).