Which backs up my point. If you have an entire species of geniuses wandering around showing off their inventions, society isn't going to look anything like a medieval world
It isn't that simple. We think of technological advancement as being inevitable -t hat every good idea that can ever be moves throughout the world quickly as soon as it is discovered. The actual history of technology does not support that view. Gunpowder was first developed in the 9th century (so, somewhere in the 800s), but its first military use was in 1000 AD. So, maybe 150 years later. A combination of it being impure, not having the appropriate metallurgy, and simply
not thinking of it meant that gunpowder was a curiosity for a century an more.
Gnome clockworks are small and break easily. It may be that the world does not have the metallurgical skill to make them large enough to do any real useful work.
In D&D worlds, races tend to be pretty isolated. Maybe gnomes are rare in human lands, so there have been few such items in human lads, much less ones that land in the hands of those with the skill to reverse engineer in under the 24 hours before the thing breaks and isn't so useful.
In the real world, making clockworks requires advanced abilities in mathematics, fine measurement and geometry. So, maybe, even if you could make an exact duplicate of one device, it doesn't allow you to generalize and make other, different devices, because the math to make the gears doesn't exist except among the gnomes, and they do it in their heads and arent' teaching anyone else.
You can use *any excuse you want*. Or none at all. If no PC ever asks why, you never have to stipulate why.