Most importantly, it's not unique.
I absolutely agree - it's not unique if you hear it on a daily basis. Which makes me wonder who are all these fans constantly telling Shadowheart "You're beautiful"?
Her saying "I know" is a direct jab at you making such a unoriginal compliment.
So let's say you're single and alone like Shadowheart. You're walking down the street and Margot Robbie jumps out of a limo and gives you the most unoriginal compliment "You're beautiful." Are you really going to just say "I know" ? Of course not. You'd cling to that moment of attention and try to build a connection - unless, of course, you already have a lineup of beautiful women to choose from.
A first date should be about getting to know each other more.
Only if you met online. In Shadowheart's case, we're supposed to know each other quite well by the time the tiefling refugees arrive at our camp.
Saying "You are beautiful" is not even close to saying "You're special to me"
It is a barebones compliment. It is not deep, it is not meaningful.
Context is crucial. Sure, you can say "You're beautiful" to anyone - even a colleague at work - and the whole spectrum of responses from "Oh, thank you!" to "Say that again and I'm reporting you to HR" is completely valid and acceptable in that situation. But a BG3 date isn't some college party hookup - it's about choosing your closest and most trusted companion. In this context "You're beautiful" isn't just a compliment - it's an intention.
Sure she doesn't reciprocate, but not all compliments need to be reciprocated
Absolutely agree - there's no need for it to be reciprocated. She just goes her way and I go mine after that. Which ends up creating a lack of female companions to romance in BG3.
and simply being interested in sex
I really prefer idealistic love over casual sex in fantasy worlds - after all they're called fantasy for a reason.