Yep, the lack of a day/night cycle is such a huge missed opportunity.
As others have said, day/night has such an understated impact on immersion, and the mechanics of light in D&D mean that night time actually has some really interesting gameplay implications.
A day/night cycle and an Elderscrolls-esque "wait" feature could make for such engaging decision-making with regards to sneaking and darkvision. It could also be tied to the short rest mechanic, which currently feels completely immersion breaking (I hated the bed-roll in DOS2).
Day/night as a component for limiting long rests feels both realistic and in-keeping with 5e rules (and frankly the rest system is probably my biggest concern with regards to gameplay in BG3 at the moment).