Q: Why are there so many stupid people?
(Please don't try to tell me you've never, even in the privacy of your own skull, asked this question ...)
I have a speculative answer:
Edit: No, forget that question. It's misleading. Pointless. And everybody in the comments thread is completely failing to understand the point I'm trying to make below. Let me re-frame it:
Why is the human species only as intelligent as it is, and not more so?
We are hominids. One of the things that makes us different from other primates is that we have language. Language enables us to communicate about our environment and to communicate our interior states. This is a very powerful tool; it means that if, for example, you have figured out a better way to peel a banana, you can tell me about it, and I can acquire that trait.
Our ability to exchange extended phenotypic traits without genetic exchange (thank you, language faculty!) makes us, as Dawkins pointed out in the 1990s, exceptional.
Because of this ability, we don't have to invent everything for ourselves, individually; we can borrow one anothers' good ideas. So we only need to be smart enough to understand and use the cognitive tools created by our most intelligent outliers.
Let me re-formulate that hypothesis: The evolutionary pressure selecting for general intelligence (to the extent that general intelligence exists) breaks once a species develops language.
And a logical corollary of this hypothesis is that we are only just smart enough, on average, to be capable of horizontal transfer of memes. Once language and culture arrived (note specialized usage of term 'culture'), we didn't need to get any smarter: we could "borrow" from one another. Therefore we're only just smart enough to do this.
(I call this Charlie's Anthropic Stupidity Hypothesis.)
Random second-order question of interest to me as a practitioner of science fiction: we can observe other species with problem-solving and tool-using capabilities around us. Indeed, the general level of intelligent behaviour visible in our biosphere appears to have been rising slowly ever since the Cambrian age. Is it possible that, if we disappeared tomorrow, new species could evolve in the post-human biosphere that lacked the language faculty but were brighter than, for example, today's primates, corvids, or cetaceans? And if so, is it possible that at some point, another species might develop language (and thus the ability to transfer memes/behavioural traits horizontally) from such a base and be fundamentally smarter than we are?