It is not a good idea. If they tried something like this, players would feel they had to hurry, and most likely most people would hate it. It's not without reason that it's hard to find a game that actually introduces such a mechanic. Most players prefer to play at their own pace.
Kingmaker didn't have any "hard limit" on camping (Pathfinder's equivalent of long rest) but it was absolutely brilliant at giving you plenty of reasons to not abuse it. Well, sometimes it was brilliant, at least. Other times it was admittedly a bit of a prick.
Maybe it is, when someone mentioned the time limit in pathfinder I took a look at the forums and it wasn't the most-liked feature and this is still quite a niche game. I can only imagine what would happen on the bg3 forums.
