Bruh, it’s an Eternals movie starring a dozen new characters all with fantastical powers. They don’t need one more unrelated superhero that the general fanbase has no reference for just there and established already. Dane Whitman is pretty clearly our audience perspective character, and then when he becomes a hero too its a fun twist.
Wouldn’t surprise me at all to see him become Black Knight over the course of the movie but its a weird stance by you. Monica didn’t have powers at the start of WandaVision. Hope wasn’t Wasp in Ant-Man 1, Rhodes wasn’t War Machine in Iron Man 1. If you think this movie is gonna be about the superhero Black Knight meeting with all the Eternals, you’re gonna be disappointed.