Artifact armor has unique models whereas AF weapons do not. To me this is the critical issue. Who cares about a throwaway item that doesn't even have a unique look? The appearance of artifact armor defines the jobs that can equip it. Artifact weapons have the same models as trash weapons that you can buy from NPCs.
Also, while it's true that artifact armor is ~15 years old, SE had already set a precedent for upgrading the armor in Limbus. These armors have already been established by SE as something they want to preserve and continue to upgrade.
Finally, again what part of the player base are the weapons going to be for? Are they going to fill a niche and actually be useful to people (maybe but unlikely since they will be in the weapon slot)? Will they be too weak and and not worth the trouble (likely)? Would SE completely go the other way and overpower them (unlikely)? To me, I don't think it's worth the effort.