Here's a bunch of rambling thoughts. feel free to tell me I'm wrong.
I would posit that not all that many people are stupid. BUT, I'd like to point out that everyone here is posting on the forums of a highbrow gaming website, on the internet, and doing so with more-or-less proper grammar and punctuation, sharing our experiences regarding stupid and not-stupid people.
Most people don't do this. There's more to "seeming smart" than just being clever and quick when faced with a problem. Most of us are probably nerds of one stripe or another; intellectuals by default. We have the patience to READ things. We're interested in stuff.
What's more, we have centuries of records that reveal the inmost thoughts of smart people, and thus other smart people can begin to see the inside of other smart people's heads. We don't know what its like to be stupid, because stupid people don't/can't write very well.
(Now, to be fair, I know many people who are by no means stupid, but type terribly. It makes them SEEM stupid unless you talk to them in person.)
Sometimes I wonder what it's like to be stupid. The best I can come up with is that the world must be very boring. (this is partially based on a job I used to have working with the mentally what-not-ed, most of whom weren't "retarded," but did have stunted intellects in addition to their other problems/disorders.) The world moves by without a handle to grab on to. You can flip through the tv channels and pass by 30 rock or Carl Sagan's Cosmos because those are "boring." They're boring because you don't understand them, and thus must default to Dog the Bounty Hunter. The bookshelf is a massive stack of boring paper full of unfamiliar words and situations that are hard to visualize. Everything goes over your head, apart from the simplest, easiest, lowest-common-denominator media. When faced with "nothing to do," you're not capable of entertaining yourself. You can't write, or think about things, or build anything. Nothing interests you, and you lack the imagination to seek out something that does.
I myself am easily bored, but because I've had to live with that my whole life, I take steps to make sure I'm never trapped with nothing to do. When I was young, I ALWAYS had a book on me. Now I've always got a smartphone handy with audiobooks or a podcast or SOMETHING. At the very least, I can work on some "mental project" or other. Right now, as I type this, I'm listening to a youtube video of an interview with christopher hitchens. But my boredom problem would be much, much worse if I was incapable of understanding, say, Evolutionary Biology or was otherwise constitutionally incapable of enjoying reading about it.