Because it's the Baldur's Gate setting and if they toss everything out the window, they should have created a new franchise instead.
You mean the same Baldur's Gate setting where you are the spawn of a god, brother to the main villain of the game, who obviously is the son of the same god, because you know, sons of gods casually stroll about by the dozens any tuesday night - and if you don't stop your brother, he'll become a god himself even before he or you hit level 7 or 8. Talk about tossing everything out of the window, because honestly in the typical DnD games I play, by level 7 I'm fighting kobolds or bugbears, and definitely feeling very mundane at that.
Honestly, I find perfectly canon-worthy and credible to be captured by an illithid looking for thralls/vessels for its species and being teleported in a hurry (and probably mistake, as it happens after the dragon breathes fire and makes the tadpole pool explode) into the topmost and most accessible layer of Hell. The fact that a blood war is going on there is taken for granted, as that's what's going on in Avernus pretty much 24/7.
The setting is fine, as for example the PnP canon campaign Descent to Avernus begins with level 1 characters learning that a whole city has sunk into Hell, and they are the ones who have to fix it, and Storm King's Thunder canon campaign has giants on flying castles bombarding the starting settlement when again you are level 1, while you will be riding a flying castle yourself by level 3-4, just to mention a couple.
We could nitpick, but I'm happy with BG3 taking an epic story arc, just as its prequels did.