It's simply a case where your AI companion makes the choice before you. As an example, a conversation where Sebille would normally have 4 conversation options, including one that she is very passionate about (and is potentially unique to Sebille). When she is your companion, and you reach this point in the conversation, Sebille interjects her favourite response before you are given the option to respond.
So this is one character in one dialogue, already requiring a script.
Now do the maths: the game will probably at least have 6-8 possible companions and hundreds of unique dialogues.
That means that Larian would have to create multiple scripts for hundreds of dialogues. I don't call that simple. I still call it a shitload of extra work.
I do think that Larian will do stuff like that for special dialogues that are important for the main quest or that offer special decisions for you to make within the group. But I'm sure this AI-controlled dialogue system won't happen for each and every dialogue and each and every possible character. If they do it, great. I'll be the first to applaud them. It's just that I don't think that it is a realistic vision to have at this point in development.