Well, in the intro of the game, we see the Nautiloid (the illithid ship) capturing people in the streets of Baldurs gate. So it makes a lot of sense that many people came from baldurs gate. That part is covered. Karlach is more of a stretch. But considering that all that happens starts with Gortach (even if later his plan was used by the brain), it seems that it is not a coincidence, but a design on his part. And later another oportunists jumps on it.
But yes, Illithids tadpoling everyone is kind of the point. Because it is not just the companions, but also plenty of NPC that cross your path.
You misunderstand Your companions all being tadpoled isn't the questionable part.
The cult of the Absolute and the True Souls are said to be based around the Western Heartlands by the game. Do you know how big of an area that is? That's the entire Sword Coast region, south to Amn and north to High Moor. That includes Cormyr, Tethyr, Calimshan, the Storm Horns the Lizard Marsh, Waterdeep, Neverwinter and more.
And yet all of the people who were tadpoled on the ship you were on all have ties to Baldur's Gate and reasons to go there to foil the bad guys REGARDLESS of the tadpole in their heads that is the questionable part. SH's Mother Superior is in BG. Wyll's dad is in BG. Karlach's seller is in BG. Astarion's master is in BG. If SH succeeded her mission, then the Gith would have followed her to BG. Gale can't just be a chosen of Mystra dealing with the problem because Elminster is busy who got caught, he has to have a tie in to the Crown of Karsus, which happens to be in BG.
That's before getting to all of them being young, fit with the same theme to ALL of their backstories...
