Because she is the game's Mary Sue and Mary Sue needs to be yearned for by the hot guy. *coughs*
Yes, she is a Mary Sue but she's the only one who does the yearning actually. There is not even one dialogue or banter that suggests any interest in her from Astarion. This banter speaks for itself:
Karlach: I never was a sucker for a smooth-talker... but I admit Astarion... you're... pretty slick. Astarion: And you're rather the opposite of slick. Do you have a point? Karlach: I was just being nice. Astarion: [Gasps] Step one of starting a conversation: Think before you speak. Karlach: Never was my strong suit.
Astarion doesn't like stupid people and makes fun of them whenever he can.
