Who the heck are you, that you know all that stuff better than Larian does?
I actually don't know that stuff better than Larian does. But if they can't add something as simple as something that already exists
https://www.youtube.com/watch?v=GgrkKzEPfbE(wait for the very end, the night is beautifull), and throw in a "is_night" variable of some sort, and dynamically change the vision ("sight"_variable) of creatures depending on some tags, well, I still think there is an issue here. Even more if you consider that after years of development, there are still thousands of bugs, then some things are wrong.
Again, I am not talking about complex NPC schedules, but really about a simple day/night cycle, with some basic effects.
If a studio can manage to find ways to finish the game while killing everybody, if they can make so that the melee can also set oil aflame with their flaming Swords, they should be able to make a basic day night cycle.