All of the Bhaalspawn are dead. This was necessary for the rebirth of Bhaal, which happened in the table top adventure Murder in Baldur’s Gate.
You're comparing board game logic to computer game logic. Don't assume that will apply here. Nothing in Baldur's gate computer games has ever given any indication of ALL children of Bhaal being wiped out. For we know we could be playing as one. I highly doubt this tabletop game you mention even mentions the main character in BG2. Who is of course a bhaalspawn. And I really don't see how ALL children of Bhaal can be found and hunted down if there's so many (let alone THEIR children to boot). You can follow a trail of destruction of course but what of those that keep a low profile? Or even go out of their way to not kill? What if you just THINK you got them all but haven't? No, that seems unlikely. But this isn't necessary for Bhaal to resurrect.
Bhaal doesn't need ALL Bhaalspawn to resurrect. He simply needs enough. "A good chunk". That's his failsafe. So even if/when he resurrects there's every chance we'll be playing AS one. Which will further make the encounter all the more interesting. The "lust for blood line" at the end of the early access version of BG3 indicates this might just be happening. If not the character then perhaps whatever sent the parasite. Mindflayers deal more with "converting" then "slaughter".
It's all speculation at this point. All we know for sure is that Bhaal is involved somehow. The absolute is likely some shady organisation that either banded together under bhaal (knowingly or not) or some resistance movement against whatever threat Bhaal presents even if he has yet to present himself "in person". The absolute isn't ONE person. It's a gang/group. That's why the goblins fear you're with them. And the drow with the goblins seems to be one of them herself. She's not THE absolute. She's ONE absolute. If it was just one person then the goblins would never call YOU the absolute. You don't just know someone and then look at a stranger and go "That's who I know". But it's very easy to do that when it's a group. That's why it shows you the mindflayers and gnolls and drow. And mentions "unity". For whatever reason they all share the same goal. Whatever that goal is. Which likely involves the parasite somehow.
