And why should the word "Relic" in this game mean the same thing or have the same importance of another "Relic" from another game? (which I'll shut the name)
You only say this because of what you've know from somewhere else.

Think "FINAL FANTASY XIV". The word "Relic" is just a word, it's just the name they've given to these weapons. See the word "Relic" as a unique feature of this particular game, don't refer to other games.