The least interesting part of the game? I've played through GTA V ~2.5 times now, I really enjoy the story missions and finding fun side quests to do. The sandbox side of the game is far less interesting to me as it's unstructured.
Sure it's not the most immersive game, real RPGs are much more so, but it's fun. I'm looking forward to playing through another story, finding more funny side quests, etc.
I don't think I have ever finished any sandbox environment game. I've played several, hundreds of hours into each. Just... happy with just screwing around, bored with being told what to do.
It's interesting, I think this is pure opinion. I personally find sandbox games boring and have never put more than an hour or two into one. I feel like there's "nothing to do". This is obviously silly, the point is that there's so much to do, but I think the lack of structure means I have no goals and whenever I try to come up with my own they're often not conducive to fun gameplay. However, when I play a story driven game, or even when I play a sandbox game with good _direction_ (Satisfactory comes to mind), someone else has curated that goal for me in a way that encourages fun gameplay.
This is not to downplay or invalidate the fun you get from sandbox games, they're clearly a very popular genre. I think they just appeal to a different sort of person.
Sure it's not the most immersive game, real RPGs are much more so, but it's fun. I'm looking forward to playing through another story, finding more funny side quests, etc.