I don't wonder. I see it as one of the human brain's greatest weaknesses. More and more research shows that once people pick a side, they are highly likely to dig in and contrary evidence actually reinforces their incorrect position. Perhaps this served some evolutionary purpose (you only need to learn fire is hot once)
Probably more local diversity so we don't get wiped out by spurious reasoning, mono-culture or get stuck on some local maximum. Instead of risking the whole tribe jumping on what they think is a good thing we'll divide into camps with the old ways and the new ways like a primitive scientific experiment. Today we don't have that strong evolutionary pressure but back when people would starve and freeze and die from all sorts of injuries and diseases I imagine this could be rather important in a shifting environment with droughts and floods and heat waves and cold waves and packs of animals coming and going could change the optimal choice quite often. Perhaps we had an evolutionary need to have people stick with what's worked in the past even if it doesn't seem to be working right now.
The other part might be that we're used to people having an agenda. The more persistent people are to convince you something is true, the more skeptical we get. There might be a value to having made up your own opinion rather than to take someone else's, even if it's wrong. That one seems even more relevant today, since more and more of what we do is make ourselves familiar with second hand knowledge, things others have found out and put to paper. There's so many tons of it you just have to accept you barely have time to get a tiny glimpse of our collective knowledge. And let's face it, a lot of that has been fantasy and fiction. You can't see AGW, but people say it exists like they used to say dragons exist. It's hard to know what is actually facts.