It isn't so much that we all know a few people who are otherwise sound of mind but believe in one of these, it is that almost everyone we know believes in one of them. And to make things worse, a lot of the people who would go 0/20 or disbelieve a given conspiracy do it for the wrong reasons. I get involved in those as third wheel a lot because friends know I'm a scientist and will ask for my input when they argue about them. Also I have this innate empathy thing going on that means when people are talking with me face to face they end up thinking I agree with them even when I've just told them I don't over and over again, because I always see where they're coming from even if they themselves don't. So they ask me to weigh in and I get to regularly find myself with one person who believes in something backwardness and another who doesn't, but doesn't for dumb reasons, the "Adam's Rib Stance" I tend to think of it. The other regular one is thinking people are morons, so at 'chemtrail' argument the person who didn't buy the conspiracy went into great - and terribly incorrect - explanation of what contrails were only to be put down by the conspiracy nut who did understand what contrails were, explained them correctly, and said they just think they the gov't uses them to deliver chemicals too. Its awkward when you have to tell the seemingly sane person that the nutter is, in fact, quite correct about what a contrail is and that they themselves had been spouting utter gibberish.

Oh, I didn't mean that apparently smart and educated folk would have done better, I just meant I expect most people with a science/geek background to assume that Earth is probably not the only place with life, even sentient life, on it in the Universe, and that people with degrees are probably disproportionately going to meet that criteria. Reason is usually just a handy horse for people to arrive at their desired destination faster and with less chance of being discouraged along the way.