Yeah, I undertand the XP splitting, but I suspect it is not quite that linear in practice? A large group of goblins (if they win initiative) could still take out a 4th level party for example. I've been surprised (weirdly) once or twice and quite badly mauled by low level goblins before I even got a chance to counter attack (I was surprised rather than them...somehow). Maybe it's not an issue....but I would rather they just say 'hey, parties are now six!' and give us a chance to play test that. I would enjoy playing with 6 party members - did that in BG/NWN/PF:Kingmaker etc. Perhaps they are concerned about not having enough origin characters, if they don't allow us to roll up more than 1 character?
Sure, it's likely not linear, but it's much easier to tweak a single formula than it is to adjust every single encounter in the game.
Currently there are 5 origin companions, so you only need a single OC to fill up the party. Or you just play with <6 characters for a little while, which is not that terrible (especially with scaling exp). Or Larian just needs to release one more origin companion, which isn't unlikely.