Why are Criterion dungeons so hard?

[Arohk]

Again, my point is merely that if it was meant to be a sort of alternate or independent content of a fixed difficulty, its gearing would likewise be independent and fixed.

You can clear it with crafted gear, if that is not good enough for you, go do savage and get that fine loot.
There is already a lack of content where your gear matters.
Dungeons scale you down, fates scale you down, your savage gear is only good in savage content, there is only a lack of rewards in Criteon which results in not many people actualyl wanting to clear it.