@Panma
I think a lot of a these generic fantasy manga are modeled in essence off of Western RPGs - they're intended for the audience to experience a world and a role in that world, without ascribing too much of a personality. Think of Bethesda games.
In those kinds of games, what do you do? You do quests, because the game designers need you to do something in the game. Same thing in this and in similar manga - the MC just does whatever people tell them to do, because the author needs stuff to happen for there to be a story.
Ironically, this is in contrast to JRPGs, where a strong personality is given, that can actually drive the story. You can think of Final the Fantasy series, Nier Automota, and so on. "Tensei Slime" would fit much more neatly into this category.