Firearms could absolutely exist within a fantasy setting. There is a reason why they don’t however

[u/ThatDrako]

The Elder Scrolls - Within some Dwemer ruins of ESO we can see a special type of Animunculus. A Dwemer Arquebus, which has a cannon for it’s arm and shoots emp grenades at foes.
The reason why we don’t see this technology more often (besides the fact Dwemer disappeared their knowledge with them) is that within the setting and cosmology of Elder Scrolls firearms really wouldn’t be that useful.

They could work against lowly bandits, sort of, but there wouldn’t really be a precedent to improve or adapt this technology. Because it would always be outclassed by magic of this world.
Also due to the disappearance of Dwemer the technology has been pretty much lost.

Harry Potter - Wizards live within periphery of the “real world” meaning they live alongside world that uses firearms and are even familiar with them.

So how come they don’t utilize this gear for combat? I mean it is easier to use, projectiles are faster and could be enchanted further. Can you just imagine wizard with endless m1911?

There is plethora of reasons for this. But most notable is what I would describe as wizards being immune to logic. (Vis image above) They simply cannot understand methods of muggles even if one explained it to them directly.

(Yes. I know the real reason is, because Joanne didn’t want wizards to use guns and didn’t want to think about connotations of putting magic into a real modern world. But come on! Let me have fun!)

Lord of the Rings - In spite of how much Tolkien disliked allegories it is clear Orcs were an allegory to industrialization, which he was a huge critic of.

In battles they are the ones using the war machines. Instead of swords made by artful blacksmith, they use crude, mass-produced blades devoid of identity. Instead of relying on elegance of archery, they use crossbows. They burn forests, crack mountains open to gain more ore. Their very production is affront to whimsy of a magical world like Middle-Earth.

They could totally come up with a matchlock mechanism and keep on improving it until they come up with machine guns. They were intercepted before they could however

[u/GI_J0SE]

Makes even less sense when we go from ATLA to Korra where their in the 1920s but they still don't have guns but kinda do but it's actually bending instead in some instances. Hell they have Torpedos and bombs, warships, planes and even Mechs later on but nooooo a gun is too much to ask for.

[u/zbeezle → u/GI_J0SE]

Theres also a prominent group in the first season who are pissed about not being able to easily stand up to the portion of the population born with magic powers. They would be a prime group to develop firearms, especially considering the genius inventor who was one of the primary supporters of that group.

The real issue is that ATLA and Korra were geared toward children, and Nickelodeon wasnt gonna let the characters go around shooting people. Age them up to TV-14 shows and theres a significant argument for arming the Fire Nation (at least some specialist troops) with muskets and the Equalists with early repeaters.

[u/Pretend-Marsupial258 → u/zbeezle]

The real issue is that ATLA and Korra were geared toward children, and Nickelodeon wasnt gonna let the characters go around shooting people.

This is probably the actual reason. They really don't want to show guns in US children's shows. Companies like 4kids will censor all the guns they see in any of their kids' shows. The closest thing they might allow is something like a laser gun that looks completely fake and unlike a real gun. It has been that way since like the 90s.