Alzrius
The EN World kitten
But it's not just from "our standpoint." It's how the game, and the facts of the cosmos, are written in most editions of D&D.
That's a meta-game construct far more than an in-game cosmological principle.
The game is written in terms of having a focus on the mortal world because that's what most gamers are inherently familiar with, and will want to play in. A medieval-fantasy world is the standard, and is present in most high fantasy media, from Lord of the Rings to Final Fantasy, and so the game is naturally constructed to appeal to that type of setting and play-style.
That's in no way an indicator that the mortal plane is the most important plane, with all of the existing primarily in terms of how they (and their inhabitants) relate to it.
There are some in-game indicators of the mortal realm's importance in the overall planar structure, certainly. Gods require worship for sustenance, and so have machinations among mortals. The flow of souls goes to the Outer Planes from the Prime (though, as Shemmy noted, they don't originate from there, but rather "ripen" there). But neither of those things explains why fiends should have any greater desire to torment mortals than, say, angels, or for that matter, each other.
but at its core, the game needs to support the basic archetypes on which it's built.
Well clearly that's not true, or Planescape wouldn't have been as successful as it was.
My point is that the source material that puts the mortals at the center of everything (which both predates and postdates the interpretations that don't) is, IMO, the better/more interesting/more mythically archetypal one.
That's your opinion, and we'll just have to agree to disagree on that one.
I can understand where you're coming from with the idea that fiends should be evil to the exclusion of all other "humanizing" factors, but I find that to be limiting, both in-game and out-of-game.
It's limiting in-game because it naturally reduces the roles you can have fiends play. They're locked into a single mindset (with minor variations for, say, alignment, or particular fiend type, such as succubus-evil versus balor-evil), and so lose a great deal of individual identity and motivation. I think that quickly makes them cardboard cutouts, which isn't what I want my monsters to be.
It's limiting out-of-game because, given that we humans are the ones playing it, trying to adequately portray a truly non-human mindset is very difficult at best. Maybe this says more about me than the point I'm trying to make, but I don't think I could adequately impress upon my players what it means to have a creature who, for all its power, intelligence, and eternal life -to say nothing of being able to access the wider cosmos - is content to keep doing the same thing over and over on such a small scale.