Because it's really not that important, you're giving it too much weight.
For me it is very important.
Would you not find it more immersive and dramatic, when e.g. you attack a bands of goblins, and it starts to rain and a thunderstorm roars over you, while swords clash and spells are cast? It can be pretty cool. Always sunny gets borring fast.
As for Astarion:
Obviously you like that character, and he is (somehow) important to you. Not for me, I don't care about that companion. But you will not hear me saying, that it's not important, you are giving this character too much weight
You compare the mechanics to the character LOL. This mechanic won't change the storylines.
I agree that it will add atmosphere to some situations, but it will not affect game that much. I will forget about it after 2-3 hours gameplay, only if the weather conditions do not affect the fight, for example, on the water you can slip or something like that. Then after 10 hours I will hate it xD.
The game has more significant factors, and by the way, characters are one of those factors. And this is about any character, not just Astarion.
It's just that I do see a lot of complaints about the day-night system in each topic, as if it would really make a difference. But apart from the atmosphere, it probably won't affect the gameplay in any way, and you talk about it as if it's a mortal sin of Larian and spoils the whole game. It's a little... well, you know... ghh.