In DOS2, you encounter the companions as normal npcs that you cannot recruit initially. Then you encounter them again multiple times thereafter and each time you can choose to recruit them, do quests for/against them or just ignore them altogether. They 'live' in the surrounding world and feel part of it
(...)
On the other hand, in BG3 it doesn't seem like the companions exist beyond the point of being your party members. You don't get to know anything about them before you recruit them and even after you do, they treat you with disdain until they flip to desperately wanting to suck your d***. There's no foreplay, instead it feels like a passive aggressiveness that you have to constantly deal with in order to find out anything endearing about them.
(...)
Minthara and Halsin are an exception however since you get to learn about them for a significant part of Act 1 and Act 2. You can work with them or thwart them, even decide on their fate. Which is why I grew attached to them before I could even recruit them.
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And having got so fond of Minthara and Halsin that manner, I still can't understand why Larian doesn't want us to have them both in the same party.
Couple thoughts.
First of all I feel you are looking at D:OS2 a bit with rose tinted glasses. Not that your feelings are wrong, but I think it's been your imagination feeling a lot of gaps. The biggest difference between D:OS2 and BG3 is that companions in BG3 have a decent amount of content.
As to BG3 specificely.
It seems to me you have an issue with origins rather than how companions are done. Acting as playable character does put them in an awkward spot, as far as they companion status goes. Halsin and Minthara feel better (well, at least until they are recuited. Halsin didn't make much impression on me afterwards), because they are just recruitable NPCs. As such they are closer to what a companion usually is, and narratively feel more natural.
As to why Halsin and Minthara are exclusive - this is pretty much the reactivity to consequence of saving/destroying grove. They are hostile companions, like Edwin/Minsc in BG1.
