How about because Relics were introduced in 2004 and it's silly to still have them as the ceiling for performance?
I think SE is handling the situation just fine. The people who already worked on them can upgrade them and still be relevant while others are better off getting the Empyrean (or in the case of a few jobs Mythic). It's taken us 8 long years to break the strangle hold that Zilart-era gear has had over the game. It's not like any kind of majority of the player base owns a relic or mythic either.
The fact is SE got it (mostly) right this time with Empyreans. They're very powerful (most of them) and a challenge to obtain but still far more accessible than Relics or Mythics ever were. I think SE could ease up on the difficult for the lv 80 stages somewhat but otherwise they're fine. Amanomurakumo is actually relatively equal in terms of power with Masamune at 90, save for Masamune's aftermath.
With the coming adjustments to older content I imagine Relics & Mythics will become easier to get too, so wait and see. Personally I think the 99 versions of all 3 will be mind-blowingly strong.