They exist but are mechanical novelties, usually just expensive wands in their own right.
Non-story reason: I hate dealing with them rules-wise. The rules will never square with what players want them to be because they want colt .45s in the era 'fumbling powder into a cannon on a stick' and don't really like that enemies are welcome to also bring guns to a gunfight.
Story reason: The role of an expensive, flashy, easy to use, armor piercing weapons is already filled by wands and spell-scrolls and other spell-storage devices. There is just no soil for that innovation to grow into something that can enter mass manufacture. You might hand a warlord an early gun, but he'd look at you like you were crazy once you told him that it was expensive, required complex manufacturing, would fail him once it began to rain, and that he'd better get comfortable collecting bird poop for the saltpeter. It also just wouldn't impress him as much. A loud bang, a cloud of smoke, and an armor piercing projectile are great, but it's not the weapon of shock-and-awe in a world where there are fireballs and lightning bolts cruising around, nor quite the leveler in a world where cavalry armor can keep up via magic and lighter materials.
At least in my setting, a weak wand is just as trainable as a arquebus, and carries far less logistical considerations on the battlefield. They're more reliable, easier to carry, have wider variety of functionality, and just more "comfortable" to the elites who enjoy that they can make one with a power that reflects their higher status and wealth. There are artificers who are tinkering with highly custom models, but they're complex and magical enough to just be seen as big wands with a lot of magical gizmos that require a full-time mechanic to operate, not a practical tool of war.