I won't presume to speak for "all the people," but I'll try to articulate my own position regarding this . . . thing. Leveling mechanics in many RPG's tend to strike me as artificial and immersion breaking. I understand and respect that leveling is part of the RPG genre's DNA at this point, and I don't mind it when there's a narrative rationale for using it. If your character is young and inexperienced, or if your character suffers some trauma that renders them incapable of recalling their past, then I can tolerate leveling up from nothing because it's part of that particular character's narrative arc. But by the time we reach TW3, Geralt's story is no longer that of the amnesiac, and he's by no means young or inexperienced. He's a veteran witcher who has recovered his memory. The narrative implications of that make it impossible for me to register the need to level up, or gather basic alchemy formulae, or scavenge weak armors, as little more than a verisimilitude-shattering concession to a genre convention. You have to level up in this RPG because that's what you do in RPGs, rather than because that's what it makes sense for this character to do in this RPG. It reminds me over and over again that I'm playing a game, instead of letting me become fully emotionally absorbed into the story of a man's search for his daughter.
If NG+ allows me to replay the game while retaining my levels and gear, I get to play the game as I would have preferred to play it from day 1: as a veteran witcher with access to decent gear, and in possession of a believably robust body of knowledge about how to do what witchers do. I get to look like a veteran witcher, swordfight like a veteran witcher, cast signs like a veteran witcher, mix potions like a veteran witcher, all from the beginning and without having to learn those skills, which is ok with me, because a veteran witcher is exactly what Geralt is. Some people will protest that this removes all sense of progression from the game, and that's a valid complaint if those types of progression are the ones that bring you the most satisfaction. But the progression that brings me the most satisfaction has nothing to do with leveling up. The progression I'm interested in is narrative progression and character progression. It has to do with making choices, exploring relationships, coming to terms with past tragedy and trying to prevent them from recurring in the future. You know, story stuff. And I am best able to concentrate on story stuff when I don't have to be distracted by satisfying a semi-arbitrary level requirement in order to actually look like and fight like the kind of character I'm supposed to be already.
That's the best explanation I can manage right now. NG+ isn't for everyone, and those of us who advocated for it weren't always doing so for the exact same reasons. We're not a homogenous group with uniform preferences. But I think that most of us who asked for it did so fully cognizant of what we were asking for.