For me it's because the back stories are way over the top. I've played tons of RPGs in the last 30+ years, disliked some of my companions, but liked most of them. I find it fun to interact with the current bunch, but it's hard to actually like them. They are are either super outgoing and friendly (Gale, Wyll), or in the arrogant to obnoxious realm (Shadowheart, Astarion, Lae'zel). Because of the game mechanics (when we meet, we are level one or two), Wyll can't back up his bravado (Blade of the Frontier, Living Legend, yeah right), and Gale's backstory just isn't believable.
You can't have Astarion *not* having trying to drink your blood, and to me that's an immediate disqualification. Not to mention that when you meet him, he immediately puts a knife to your throat. Lae'zel treats you as if you are something she scraped off her boots. I can actually relate to Shadowheart, because a lot of what she is putting up seems to be a front.
I just can't see why the main character, e.g. me, would be running with this bunch. The way they are introduced, I wouldn't invite them to join me, with the possible exception of Shadowheart, and her only if I tried to rescue her beforehand on the ship.
In 4 characters, I've only had him put a knife to my throat once.
I can make my determination on who to take right out of the gate. If I'm not on a "tanky" character, I can keep Lae'zel. If I'm needing the services of a healer more often than I'd like to admit, I can keep Shadowheart. If I'm not a rogue, and think I'll need one, Astarion is on call. I'm not meant to get all the story beats in one playthrough, so if I decide one of them must die early on, I can do that too.
What I really hope will happen is that this will be one of those games where I spend 1000s of hours in it. Not a "one and done" game, or one I can't even bring myself to finish. With that in mind, I'm perfectly fine with not being a "people pleaser", making choices just to jack up approval on the comps. If they don't like me, so be it. If they don't like my choices, so be it. I'm not going to be fussed about it, and I'm not going to alter my plan for a character just to garner some approval. They are their own person, and that's great. I don't like everyone I meet out here in the real world, and I don't expect I'll like everyone I meet in a game either. I'm not going to harbor any resentment towards them because they don't make me feel like a special snowflake either. Truth be told, I'd rather not be a special snowflake, for a change.
I've played the trope so many times, it'd be refreshing to just be "one of the boys". That's not going to be the case here, we're going to be a special snowflake, even if I'm not sure exactly why, just yet, but the story focus is going to fall squarely on us, again. I wouldn't have a problem with being part of the solution, instead of being the solution. I wouldn't mind being the "leader" of this group simply because I was the only one willing to take up the mantle. At the end of the day though, this team is made out of the need to work towards a common goal, not becoming a mind flayer, than any sense of friendship. If it's the only way to save my ass, I can work with people I may not care for to do it. Of course the Origin characters have fleshed out back stories, they're designed by Larian, and they can be the main character, at some point. Custom characters are supposed to be blank slates, at least if my experience with 30 years of RPGs is anything to judge off of. It's amazing to read on these forums the implication that Larian should have completely written the backstory for the main character, if it's custom.