I think it’s because they were actual friends, and not some weird romanticized “will they, won’t they”.
I think the writers are so used to that sort of thing nowadays that it just infects the writing. I agree that companions are way better when they’re just friends. Ex. Rose was kind of nice because she was just a pal to the Doctor. Trying to shoehorn romance into a show where the Doctor really isn’t a romantic creature is so weird