I know all that, but I don't care. The "it's their culture" excuse only works if you believe that all parts of all cultures deserve respect. Have fun justifying
cannibal culture. Sure, calling your loved one by their last name for years isn't on the same level as cannibalism, but it
is retarded.
That's another Japanese trope I dislike. Not putting a name to a yuri couple's relationship, so they just describe themselves as "weird" or "special" or whatever. It happens because same sex marriage isn't legal in Japan and it's somewhat frowned upon, so they tiptoe around the whole concept. I also consider this retarded.
Now
this is the actual reason for sticking to calling each other by their last names.
Whenever a title contains the name of a character in their story, Japanese authors have a tendency to keep using that name regardless of whether it makes sense or not.
They may some times choose to fight this instinct of theirs, but most of the time I've seen them opting for creating retarded justifications for why it makes sense, just to keep the names the same as in the title. Or, it just doesn't get addressed at all.
An easy example where this shite happens too is
Hori-san and Miyamura-kun. They still call each other that way even though
they became a couple and had sex, just because it's still embarrassing to use their first names. Bitch, please.
You two literally fucked already! It's not cute. It's absolutely retarded.
Their relationship isn't even that abnormal in the first place. At least, it doesn't justify them sticking to using their last names.