Hayao Miyazaki gets a deserved reputation for being a cranky old man, but there are times when his garrulousness is spot-on, as when he recently complained that the big problem with the anime studios today is that they're staffed by otaku.
I agree, although I'd put it a slightly different way: the problem is that they're staffed by too many people who are nothing but otaku, and that's something I should talk about in detail at Ganriki when I'm back off my self-imposed hiatus.
I have the same issue with SF&F: it's too often written by (and maybe also read by) people who are nothing but SF&F fans.
At the risk of sounding hopelessly idealistic, consider Isaac Asimov. Here's a guy who wrote some of the most foundational (pun intended) SF around, but also wrote about everything from Gilbert & Sullivan to the Bible and Shakespeare. He was accultured and curious, and those were qualities reflected in his work. Sure, he's the extreme exception, but shouldn't that make him all the more of the ideal to aspire to?
Today's big-name SF authors seem mostly interested in science and technology, and other SF, and maybe a few social issues that cluster immediately around such things, but little else outside of that. Fantasy authors fare better — the variety of world-building involved there demands it, I suppose — but too many of them still seem to run into the same fences. Most SF&F's understanding of human nature seems to be derived not from life, or even from area outside SF&F, but from other works of SF&F, and that's far too self-limiting.
I suspect part of the problem is that there's this unchallenged assumption about why one should read outside one's comfort zone. One doesn't read Kafka to "write like Kafka", but to know his worldview. That requires good teaching, and also good critics — people willing to speak up for such work and bring it to attention in circles where it might never otherwise be considered. I suspect someone somewhere is teaching a "Lit Classics for SF&F Authors" course, but we need to be more in the habit of discussing such things amongst ourselves.