It shouldn't be and there are RPGs that aren't like that. Enemies should get progressively harder, just as much as protagonist does. When game doesn't need any involvement from the player on high level, then it failed. For me, high level combat in DA:I is plain boring and feels like chore.
Fixing DAI for people like me (who like it with absolutely no challenge at all) and people who like challenge:
#1. Casual twice as easy (enemies with half HP and doing half damage)
#2. Normal twice as hard as it is now (by creating a better levelling for enemies, optimized builds and AI, enemies have lvl 1 focus abilities after level 12)
#3. Hard twice as hard as normal should be (as described in #2) with enemies having 50% more HP, 50% more physical damage, 25% more ranged damage and 25% more magic damage, enemies have lvl 1 focus abilities after lvl 11 and lvl 2 focus abilities after level 16
#4. Nightmare should be the hard of hard, with the same changes we had from #2 to #3 and in addition focus lvl 1 after lvl 10, focus lvl 2 after lvl 15 and focus lvl 3 after level 20
Casual: You're in for the story, not the game, combat would be purely cosmetic
Normal should be moderate, not retardedly easy, but not time consuming either, each fight should count, boss fights should be hard, but not make you have to try again unless you are a newbie to RPGs or forgot to use your resources
Hard should be more time consuming, resources consuming, and have deaths
Nightmare should be about you having to try again and I think an elite rogue enemy going mark of death or thousands cuts on you would do that just fine
It would be great because until now its been:
Casual - Huh?
Normal - Did I change the difficulty?
Hard - After skyhold, casual, at best
Nightmare - After skyhold it is casual as well unless you try to make an awful build or skip crafting