No, that’s not a game’s job. You will not enjoy every game. Those games that you did not enjoy are not necessarily failures. They are simply games that _you_ did not enjoy.
I don't believe in the concept of "fun in general" in the sense you imply here. I've played very successful games that have evidently achieved popular appeal but that nonetheless turn me off because they're the absolute opposite of Dwarf Fortress: carefully manufactured designs of intentional experiences that you are guided through at every step. Some people really like that, some will want to be thrown in the wilderness with no clue, some want something in-between. Different strokes for different folks.
So are you wrong in saying generally that it's "despite its terrible onboarding, not because of it", or am I coping by lying? Mind you, Dwarf Fortress is preceded by a whole genre of more traditional Rogue-like games that made losing hundreds or thousands of times for a lack of knowledge of the games' systems before you figured them out a fundamental part of their appeal, so I'm not exactly alone in this.