https://screenrant.com/frasier-show-daphne-niles-love-not-realize-reason/

Niles has been clearly in love with Daphne since they first met, but she never found out about it until someone explicitly told her in Frasier — why?

that's the storyline. No reason to read anything more into it.

She had to have always known that he was into her on some level.

that's the storyline. No reason to read anything more into it.

Yeah. It seems to be common in later shows that use this story line - Friends, Frasier, The Office, HIMYM, Big Bang Theory - that the woman is either completely oblivious or else deceived that the guy has accepted that they're just friends.