This being a prevailing issue in many forms of media does not suddenly make it good or okay writing. Hell, every genre you've listed has never been known for smart or intelligent writing. Don't get where you're coming from in terms of it being a thing in RPGs. In almost every one I've played there is a reason the MC is the one doing these things and not somebody else.
Dragon Age? You've got special powers that almost nobody else has and are basically forced to fight the rest of your short life. Fallout 1 and 2? You're the best candidate to leave the settlement. Literally any JRPG? You're the chosen one special anime protag. KOTOR 1 and 2? You're literally the most broken characters to ever exist but just didn't know it yet. Divinity Original Sin 2?
Deus Ex? Go on missions from your boss, get wrapped up in a huge conspiracy along the way. The Witcher? Get wrapped up in political drama while trying to solve personal drama. Also there are hardly any witchers left. Disco Elysium? Who else is gonna fix your amnesia? Elder Scrolls? You're usually a part of some prophecy. VtMB? Start as a nobody, gain enough power to do whatever you want.
You get the point. Usually in RPGs you are the one in the middle of things because not many people have the ability to do what you can, and/or the MC is privy to special knowledge most others don't. In this game, other people can, according to the devs, help just as much if not more and have the same knowledge as we do.
Even if every single example I brought up is somehow wrong, I'll take that L. Still doesn't make it good writing, though. What-aboutism is like the worst defense to literally anything.
I would disagree with almost all those examples, in that most/all of them are just contrivances that don't hold up under scrutiny roughly the same as this game.
But more importantly, if you wanted instory reasons, they exist just fine in this game.
You, specifically, are after Kass because you're
forced to be.
She told you shes going to take your soul and jangled your destruction in front of you like a set of keys, slapped you with her dick and moonwalked out of the room. Your quest is motivated by self-interest.
The gods aren't helping because Kas, in and of herself, isn't a real threat in their eyes. The face to face she had with a god he told her that, point blank. In that same scene, he says the reasons the gods are keeping their hands off - they're interested in what Kas is doing.
Remember, noone has actually seen what the demons did to Mareth. We know, because of meta knowledge, but the majority of people that actually know about her (and besides the gods its a small amount) haven't met her and have no reason to think she's a particular threat to them. And even if they did, they have no reason to think they could do anything about it - the only reason she hasn't squashed YOu, specifically, is because she has a soft spot to you. The being stronger than you doesn't translate to them being stronger than her.
The priest is a pacifist who wouldn't even beat up a couple of kids.
Garth is practically crippled - he limps just getting around the bar.
Gweyr is both in exile and, strictly speaking, HAS been doing something the whole time - longer than you, actually. And afterwards, she's finally reunited with her family.
And we have next to zero info on Leofric. Who knows what he's doing and why?
tl;dr Very few people know about Kass, even fewer see her an existential threat, and the people that fall under both categories are...well, generally in your party.