That's because we run the same content and storylines in epics, obviously. There are exactly 15 epic-only quests in the game, and most of the new "legendary" raids are just continuations of heroic storylines that would absolutely made sense as heroic raids from the narrative standpoint (except Killing Time, maybe).
Had a thread here about it.
To change a monster from a quest, you also have to change the narrative of the quest. It does make a difference, if Arraetrikos lieutenant is a kobold. Because then it makes sense that its underlings are kobolds as well, instead of angels with chainsaw-swords. I mean, such angels would look like legendary monsters, but why are they following a kobold who is following an archfiend, hell-bend to use a magic codex for whatever?
Changing story is work, much more work than some scaling. As long as the devs decide to recycle quests for heroic and epic/legendary (which means 2 dungeons with the work of 1.125), the most epic thing your character will ever do is to invade the demonweb to steal Anna from under Lolth's hairy legs. Which has a base level of 24.
Because when you have the split, it's much easier to make a heroic storyline than to explain why the heck your level 6 character is duking it out with demigods. Which bring problems like
not duking it out with Hyrsam and that aside from the Formorian King, the raid brings out rather lackluster monsters.
Just saying.