NBA 2K has a career mode where you take control of a new player. You play games to raise stats like shooting, passing, etc and raise your overall rating. It has a sort of built in story where you rise from scrub to super star. No one calls NBA2K an rpg (I say everyone as like almost all people being that I'm sure someone has at one point). Despite it technically meeting some mechanical requirements.
Some racing games have you race (fight) to gain more prize money (exp) to get better stat upgrades (better parts) or gear (better cars). And sometimes has a story mode (although not always). No one calls these rpgs.
Without so many words
A game where a player takes control of an avatar meant to represent a character(s) in the game that provides a sufficient level of story telling which is presented in some sort of narrative (usually a mix of cutscenes, a DM talking, etc and dialogue with npcs but that can differ).
What is actually a sufficient level will vary, but Dark Souls has like nearly none. The only thing that makes Dark Souls anything like an rpg is that it has stats and xp. Like lets say on a scale of 1 to 100 on how rpg is a game with 1 being the least Dark Souls would be like a 20 and 3D zelda games would probably be closer to 40 imo since it fulfills the narrative and storytelling and has more interactions with the world and characters etc.
The term RPG for games is very similar to Slice of Life in anime. Technically the tag can be used for nearly every single work with a individual in it in some form. But it's not because we often have more fitting and specific genres to put things into. (And yes I have talked to people who think things like Cowboy Bebop are slice of life).
So in general I ask this question. If someone just asks me "Do you have an rpg you'd suggest" and if I would not suggest said game it's already in a bad spot for being called an rpg. Even if they said action rpg would you say Dark Souls? Because even if they were looking for something with more action oriented content versus a turn or pseudo turn based system there are simply games that fill the role much better. On the flipside if they asked you for a hard action game you might say Dark Souls.