Yeah, they COULD put a day/night cycle in if they wanted, it certainly is not beyond their abilities; and that would accurately solve the "rest when you want" problem because long rests are ( at most ) once per 24 hrs. Actually having to wait in-game before you can rest would definitely promote the proper consideration of when you use per-day resources.
But there are 2 problems with doing this. First, Larian's co-op play model ( which is apparently very popular ) relies on time being an illusion. And second, most people that buy the game would hate being made to wait and, therefore, complain; this would be a valid complaint, since having no respect for wasting your customer's time is always a bad idea if you want them to buy your product.
An easy way to have this for multiplayer would be for all party members to have to agree to rest at the same time (maybe a voting system.) I am not sure what you mean about people complaining about being made to wait though, all the multiplayer games I have played require some sort of communication and agreements between players and this should be an expected thing here.
Oh, sorry. What I meant was that MP is the main reason why there is no concept of the passage of time in BG3 ( and D:OS games) because each player can go in and out of TB mode independently; as SP uses the same engine, it inherits this behaviour.
But, if the game did implement passage of time at, say, 5 real minutes per game hour, then per 5e rules you would need to wait 2hr real play time between long rests in SP, regardless of your situation or playstyle. That would definitely provide a barrier to frequent resting but it would not necessarily be popular to do nothing for that time, while waiting to recover your spells and abilities.