Because this is their way of getting players back into old content. It's like any other part of the relic quest line, they need to force players into returning to old content because otherwise nobody in their right might would touch it.

Atma - Have Level 50 players spam FATEs to help new players have another option for EXP and make the open world seem more populated than it actually is
Animus - Do old dungeons to make queues better for new players + myth requirement means you were spamming some Lvl50 dungeons anyways
Novus - Just a huge gil sink in addition to the grind, still dunno wtf their intention was with this since Materia is ridiculously RNG and expensive as a result
Nexus - Spam a bunch of old trial instances to help new players (Though in their shortsightedness they didn't take into account people would just throw up PFs to burn through them as fast as possible rather than queuing up)
Final Stage - Do old dungeons (Wow this sounds familiar by this point), now with RNG, to help new players

The Relic Questline after the first step (Chimera, Amdapor, Hydra, Primals, etc.) was never about actually having something enjoyable to do. It was always about having a huge timesink along with making content easier to access for new players rather than making it enjoyable for the people doing the questline.