I honestly think a lot of it is brought on by making companions also be able to be 'origin characters'. This is a concept that I like, but I never quite understand why Larian has to make every origin character sort of have the same plight... or at least it is something they seem to struggle with in their last two games. Everyone needs to escape the ship, get the collar off/parasite out and deal with a very similar big bad. Then after a while their paths diverge, even though the same major overarching plot applies to them. But it seems like they could just be very different from the beginning. Like I see no reason why they have to share the burden of the collar or parasite - this mechanism is not necessary for having a similar goal toward the end of the game.
That pretty simple to asnwer. Because they are all playable, they need to share a lot of content, and because writing unique multiple playable protagonists with different motivations and objectives would lead to even more dialogue to be written rather then occasional character specific lines - that was the case in D:OS2 and is likely to stay the same in BG3. Imagine not having tadpole, and how many interactions would be removed. Tadpole, like source, act as a convenient narrative device to override any roleplaying goals player might make, and conveniently ties player character to the plot of the game no matter their origin, or imagined background. Tadpole seems to be an equivalent of source, then collar - we ain't gonna be rid of it, and likely will tie every single origin to the plot till the end of the game.
Having different origins start in different situations, could also clash with coop - do you have individual players start in different parts of the map and having to meet? Do you force them to play singleplayer intro, before starting game proper? As it is, you can create coop experience and all start as fellow prisoners. I think it's very neat for coop, a bit less exciting for singleplayer.
So that is what I was thinking, but then I thought (and again, this would fundamentally change the game, but in theory) if you removed the tadpole/mindflayers as the main story and added in something else down the line that brought them together... it doesn't seem like it would be that hard.
Default Tav could wake up on the beach.
Astarion could wake up in the Dank Crypt since he is a vampire.
Gale could teleport into the setting like he already seems to do. (At least he did when I did my playthrough)
Lae'Zel could be on some kind of quest in the area for her people or whatever.
Shadowheart could wake up somewhere nearby not knowing what happened to her.
Wyll could be hanging out in the village - like he already is.
Which is to say... as long as you come up for a reason to them all to be in the same place or near the same place it doesn't seem like it is that big of a deal. If using the existing map they could all just start out at different areas near the village and the quests could all kick off from there... and some of them could be "side quests", while others would be part of whatever the main story is. It would restrict any content, it would just change the beginning parts of the game in a minor way... and you could introduce them all with their own little side quest for wherever they start - which doesn't seem like a very big lift.
