DMs are just writers that have an idea for a story, but don't feel like writing a book.
Kind of a shower thought, and maybe it's just how I run campaigns, but it feels like there's a lot of parallels between DMing and that guy who has a great idea for a book/movie but will never write a single page.
The DM comes up with "So what if there's this guy with this magic power, and the only way to beat him is through a very specific process." And then all the Players decide: who are the characters? What do they do? What do they say? How's it all work out?
Tabletop RPGs are such a great way to bring some life to a story that you're still workshopping. Sometimes I have an idea for a story I'd like to tell, and it ends up seeing the light of day at game night before I would commit to writing it.
It's great too because it's so collaborative. The life of the story is usually character-driven, so having other people play those characters goes a long way to giving them their own voice.
You also have a captive audience as a focus group. At least, in my group they're not afraid to tell me if I have a stupid idea, and they're always coming up with outside-the-box solutions that I never would have considered.