I don't understand why for the D:OS games people love playing the Origin characters there, and how the story feels richer playing one of them, but they hate the idea here in BG3. How is it any different? And why resist the idea so much in BG3 if it already worked well in D:OS?

I for one love the concept in both games, and can't wait for the OPTION to be available. Yes I plan to play as one of the companions, after playing my own character I love the concept of getting into my favourite companions' minds a little more, so to speak.
It is an option - people who don't want to try it, don't have to. Just like people who don't want to do the Evil Path, don't have to. Etc...