Because whether or not an element/isotope can exist is a property of nature. We have to discover whether it can exist.
Manufacturing implies we can have any element/isotope we want, which is not the case. By attempting to manufacture, we discover whether manufacture is possible. We discover the existence of a new isotope.
Whether an isotope can exist is a property of the universe. We similarly discover the speed of light, discover quantum mechanics, discover radiation, etc.
Couldn’t you say the same thing about cars? You can’t just manufacture a car that gets 200k mpg. So we discovered a car that can get 30mpg. Which we don’t say.
I am not satisfied with the other answer, because while it is correct I don’t think it makes the distinction clear enough. So I’m gonna ELI5 it, dumb it down far more than you need it to be, because it gives me the chance to make silly metaphors.
The atom that they found - let’s call it Bob - was, for all intents and purposes, created.
The discovery isn’t of Bob. He’s cool and all, but he doesn’t stay around for a long time so there isn’t much that he can pers… atomly do for us.
So what’s cool about Bob that we can record and know for later? Well, there are two things to point out. First is that Bob stuck around for a while. The second is that Bob is governed by the laws of physics.
Therefore an atom with the same properties as Bob can, according to the laws of physics, stay around for a while. That’s the actual discovery.
Well, this discovery can be summarised more succinctly than “we found that things with the same properties as Bob can exist for a while”, because the name for the class of all things that have the same properties as Bob is the isotope Uranium-241.
Now, we can say that Bob is Uranium-241 just like we can say that I am a member of the human race, but just like we can’t say the human race IS me, we can’t say Uranium-241 IS Bob. Uranium-241 is the mere category of things that we know Bob was one of.
If aliens created a wormhole that accidentally sucked you away to their planet and they saw you, did they discover nonethewiser? Or is it more profound that they discovered the human race?
Same goes for this. We can say that we discovered Uranium-241 (which is shorthand for “we discovered that Uranium-241 is stable enough to survive long enough to be measured”). We did so by creating Bob, which merely acted as proof.
The ideal thermodynamic cycle was discovered in the 19th century [1]. We've known ever since why 200k MPG is impossible and we later discovered the design that evolved into the modern internal combustion engine, a process which was mostly engineering, metallurgy, and the development of industrial quality control.
Manufacturing implies we can have any element/isotope we want, which is not the case. By attempting to manufacture, we discover whether manufacture is possible. We discover the existence of a new isotope.
Whether an isotope can exist is a property of the universe. We similarly discover the speed of light, discover quantum mechanics, discover radiation, etc.