BioWare jettisoned the Day/Night Cycle when they launched TOB.
I spent a lot of time on their forums back then, back when BioWare still had their own forums, and they explained that they felt the Day/Night cycle didn't really add anything and more often than not you had players standing around waiting for it to become day so they could interact with the folks they needed to interact with. (It also spawned ridiculous situations like traveling from one map to another and it taking 78 hours and you show up exhausted when obviously you would have rested on your journey.)
It really wouldn't be a BG game if they included a day/night cycle as the original developers of BG decided that it wasn't worth it over 19 years ago. It would be a huge slap in the face to the lessons that BioWare learned.
I would beg to question whether they learned the correct lessons.
One could also say "BioWare learned the lesson that cRPGs are shit and therefore abandoned the genre after DA:O."
Once BioWare was purchased by Electronic Arts their games certainly went downhill.
For those who enjoy a Day/Night cycle more power to you. If you think it would add to the game -- good for you. However those arguing that no Day/Night cycle means it's not BG are factually incorrect. The BG franchise dropped the Day/Night cycle with Throne of Bhaal. Using a Day/Night cycle in BG3 would go against the lessons learned from the BG developers.