I think there’s a tendency for people to think that writers are particularly smart people. Having been on the receiving end of it a couple of times, I’d like to dispel this notion: no, writers are not geniuses.
(Or at least, I'm not a genius).
This is as much a reminder for me as it is a notice. I think people think writers are smart because the act of writing is an act of seeking clarity: ideas become nakedly good or bad the instant they hit the page. The essay doesn’t lie.
Unlike a talk, a conversation, or a presentation, writing exposes ideas in their barest form. The act of reading is an act of contemplation — sure, really good writers are able to convince you for the duration of their book, perhaps. I’ve had a really smart friend once warn me — when I told him of my mission to read more non-fiction books this year — that non-fiction books tend to absorb you completely. Particularly persuasive writers are able to convince you that their worldview is the Right Way To See The World, and it’s only after you finish reading the book and think about what you’ve just read that the limits of their ideas become clear.
His admonishment was a simple one: “don’t forget about the world outside the book.”
With that said, the temporary idea-bubbles you inhabit while reading are still better compared to listening to a particularly polished speaker. Speakers have access to a vast array of persuasive rhetorical devices not available to writers. It seems bizarre that merely speaking well can make a stupid idea convincing, but listening carefully to the contents of the average TED talk will make this very clear — very few TED speakers are worth really listening to.
Writers seem smarter because writing forces them to clarify their ideas. Beyond a certain level of competence, the quality of writing becomes the quality of their thinking; conflating the two is why you think writers are smarter than they really are.
Additional reading: Paul Graham writes about this effect here.