It's hard to invent new magic items with 5e's default magic item rules, because how long an item takes to make depends on its rarity! A unique prototype is going to be (by definition) at least as rare as the rarest item, so the production would take as long as the rarest items: 20,000 days (about 55 years)!Yeah it's a matter of taste and preference in world building, as well. I don't like presenting worlds wherein the past was better than the present, or where all the "cool stuff" are things no one knows how to make anymore, and prefer worlds where the magic sword that burns with the fire that forged it, glowing red hot and dealing additional fire damage, is something that the guy in the neighboring kingdom invented ten years ago. There is one of them, because your dad was the first person to ever commission one, and the method for making it hasn't been repeated yet. The occassional ancient relic is great, but I never make such things better than more modern inventions.
At least the Xanathar's rules bring that down to only a year for a prototype... but it adds in the requirement to face CR 19+ creatures.
(No I don't actually advocate interpreting the magic item creation rules that way. Just pointing out how silly they are.)