I gave a world-building-focused reply, which I don’t want to loose to the vagaries of the twitter algorithm…
Maybe they just haven’t figured out gunpowder or steel yet. Tech development isn’t like plant dev, it isn’t hard-coded into any DNA. It’s contingent: people have to want to develop it, have the resources to, succeed, & have it gain enough acceptance that it gets improved on. Like, there’s no reason that in-world metal working that created butt-kickin’ swords in game terms has created metal robust enough to use for muzzles. Yet. Magic doesn’t have to stop tech development but it might restrain it from going certain directions. For a game century or 2.
That was a two-tweet reply. Focused on world-building. But as I thought about it over the rest of the day, I also had game and mechanics answers.
The original tweet was hashtagged for Dungeons and Dragons generally, and D&D 5th edition specifically as well. So. Yeah. Firearms are a thing people have wanted since, basically, the beginning. For whatever reason. The game has rules for fireams in the 5e Dungeon Masters Guide (has them in the AD&D DMG, too) the game, as a product, isn’t super excited about them (they appear on only two pages).
There are also kind of a lot of options on DMs Guild. People can walk down that road as far as they like, and people can walk along with them. And, in walking down that road, they want to get into it about whether they’re still playing the same game. D&D is a complex enough game that in a very strict sense, it’s almost impossible to always play the same game. We forget rules or apply them inconsistently all the time. Unlike other kinds of games, the referee is also a player. There is not someone whose role in the play session is exclusively to watch for rules violations. So, if you enjoy that kind of hair-splitting, more power to you.
So the game answer to the question is, it just sort of does, because that’s the product that it is. This is, to my view, different from an arbitrary answer, or a hand-wavy answer. The game just doesn’t do firearms with much enthusiasm, but it does. The game doesn’t say “magic makes firearms not work,” nor does it say “because there’s magic in this world, nobody gets around to inventing firearms.” It’s just not a game that cares to really feature gunplay.
I also have a mechanics response to the question, and it’s one that sort of looks past the actual question, and addresses an assumed unstated assumption: that firearms model something that other combat options do not. Mechanics give us the way we talk about the imaginary space our imaginary people operate in. In what way would a rifle or handgun be different from magic missile? The question can be extended in many ways. In what way would a shotgun be different from crossbow? If there are firearms in the setting, how to they affect the relative effectiveness of other combat options?
If firearms are not mechanically particular, why bother adding them in? Often, and 5e specifically, elfgames have ranged weapons, and fire & lightening magic. Add in wands that pretty much anyone can use, and firearms start to lose some of their mechanical appeal for me. (And they didn’t have any real game appeal for me, either.)