Because it's really not that important, you're giving it too much weight.
If you are referring to a day/night cycle, then I would strongly disagree. The lack of day/night/weather totally breaks immersion for me and I find the permanent Mediterranean summer in the game frankly quite jarring. Even a fantasy game should have some basis in realism to draw you in and create a palpable sense of a living world.
Anyway, I digress. To get on topic, I don't care for Astarion at all, only took him along because I needed a rogue but I do find his story OTT. As someone else mentioned he would probably work far better as a snobby Elf noble without the extraordinary vampire backstory.