For starters, I don't think the WN ever provides evidence that any... let's use the term "mortal" currently alive actually knows the full details of the compact (including the ruling class), and/or whether any of those mortals are actually aware of the alternate world prior to the events of the series. Dragons are a something of an exception, but they basically never interact w/ the normal mortal races. I also have a vague memory that the Rabbit priest in this arc MIGHT know more (& possibly everything?), but I don't think he ever really spells things out in detail; I think it comes up at some point that part of the compact is that he's not allowed to tell the whole story? This part's very hazy.
Anyway, this ruling family IS aware that they're supposed to give this specific orb to a specific descendent & that descendent will disappear forever if given the orb (thus the king obviously wants to give it away!) BUT, I don't recall ever reading that the king knows WHY the person giving this orb will disappear and/or where they disappear to (& I'm pretty sure it was quite the opposite, they know the result but have no idea WHY this happens every few generations, which is part of his angst on why he doesn't want to give the orb to his son.) In a more general sense, I never saw it spelled out that any of the mortals know what's going on, & bc of that I don't know that the ruling class is any more or less greedy/terrible than they are in any other of these isekai settings, & if they, are I don't think it's bc of this subject.
The second thing I'd bring up is that the other world is definitely a shithole, and yes it's filled w/ terrible monsters, but I don't think I'd say those monsters "want to invade the human world", bc said terrible monsters are generally of the "Hostile Magical Critter" variety, not the humanoid/sentient beings of that world. Humans, or at least an offshoot of them, are still alive in that dimension along w/ several other humanoid/mortal races (IIRC, some/all of the non-human races there don't exist or are very rare in the main world, but this is another subject that the memory's kinda hazy on.) The members & leaders of those races ALSO don't seem know the Good World exists (bc if they did, the leaders of those races would be trying to get their people out of the Dark World instead of being in a constant war w/ the environment for survival.) I have a vague memory that the leaders of the Alt-Humans may be the exception to this in that they know the other world exists & want to get there, but they don't seem to know HOW to get across, so their lives are just as shitty as everyone else's in that world.
This is another bit possibly open to interpretation, but I feel like the implication for WHY the Dark World is filled w/ terrible monsters is that the imbalance in orbs (or at least orb USERS, more on that in a bit) has left the sentient races off the Dark World too weak to properly fight back against the monsters there, leading to a spiral of more/stronger monsters attacking the Sentients> Sentients are forced to retreat due to insufficient combat power> the resulting abandoned land leaves room for more/even stronger monsters, rinse & repeat.
As I alluded to just a bit earlier, I think a large part of the imbalance isn't just that there's fewer/less powerful orbs in the Dark World, it's that there appears to be hardly any orb USERS. When the MC ends up in the alternate dimension, he discovers that the alternate world version of the shrine they're all currently at appears to be abandoned (or close to it, I think there was something resembling a shack or the like there) & it's absolutely OVERFLOWING with unused skill orbs (so much so that they burst out of said shack shortly after he arrives.) Good-world humanity is certainly contributing to the imbalance by hoarding orbs (mostly/especially the powerful ones), but for reasons that weren't fully explained last I read, the Dark-world seems to've largely lost/forgotten that orbs are even a thing & thus almost no one has or uses them.
I've probably droned on too long at this point, but the last little bit I'll bring up is that I think the WN sets up the the true culprit (IE the parties responsible both for the imbalance & that almost no one KNOWS about the imbalance) are actually some section of the individual(s?) involved in the original compact (who are all weird supernatural entities mind you, not the mortals having to deal w/ this ish) & are doing so bc they want out of and/or to destroy/rewrite the rules of said compact... but all of this was all in the realm of cryptic hints & foreshadowing last I read, so it's possible this line of thought was eventually revealed to be moot.
