Why doesn't it?

use the extracinator to get copper

copper shortsword was chosen because its an iconic weapon (which the zenith is mostly composed of, theres popular stuff like terrablade and its own components, but also some swords that nobody uses, like seedler
tin shortword is nothing compared to copper one when it comes to iconicness so thats why it can only be crafted with copper one, though i do think it should be craftable with tin shortsword just people with tin-generated worlds don't have to constantly go from one world to another or go fishing

Yeah ok that makes sense

Also, the reason why the copper shortsword is iconic is because of it being the default weapon that you get the very moment you enter a world with a brand new character. (Doesn’t apply to Journey characters)