May I contradict you in a way you can agree with? You’re right about the dragons and fantasy, of course. Thank heavens the world is not made of nothing but memoir with everyone literally writing what they know. I enjoy memoir, but I am glad it’s not all there is.
But still, writing IS about what you know. Let’s say you are writing a fantasy and the hero is facing down a dragon. Have you ever been scared out of your mind? Write that.
Or maybe the hero just saw the princess. Have you ever seen a person who made you lose words with the electricity of that first look? Write that. If you’ve ever held someone’s hand as they died, you can write what you know when the story calls for sorrow.
Bad writers often have characters react in really lame ways — ways that would not be how a human would react in that situation, and so their writing is not believable. Writing must be believable, even when it’s fiction.
This is what “write what you know” means. People love to quote, but often do it without understanding the quote.
Write what you know is attributed to Mark Twain, which is probably why so many teachers quote it and it’s so ingrained in the literary world that 100+ years after he died, writers still argue about it.
I have not found the source of the quote to know if it was Twain or not, but if it was, here’s what I do know — Mark Twain wrote 28 books and they weren’t all memoir, but he most definitely wrote what he knew. His childhood is painted all over his books, nevermind that they’re mostly fiction.