They can. They chose not to, in order to focus on things that directly impact gameplay.
From the blog post
Here we go:
"One alternative would’ve been to put in a day/night cycle without a lot of reactivity from the npcs, just for the sake of being able to say that we’ve done it, but that’s something we didn’t want to do. The vision driving Divinity:Original Sin has been that every feature which is in there has real gameplay value and isn’t a gimmick."
