Immortal
Arcane
It's because Swen wants the night to have some gameplay element and not be in just so it can be in. And they don't have the time/money/desire to do it.
Doing only aesthetic would add lot of flavour anyway, much better than nothing. Also the engine is clearly capable of it so it's not the best motivation.
Plenty of 90's games had it
While you are correct - the engine is capable of tuning the time of day / environmental color / fog If Night / Day cycle has no mechanical or game play effects.. leave it out.
Doing Night and Day is a huge amount of effort.. Witcher 3 had to completely redo their Rendering system because areas would look good at one part of the day and horrible the next.
IMO pick the best aesthetic or mood for the area you want and put the dev time else where.