Like manga, I hate it when they, for example, transliterate さん as -san, when there is an “equivalent” word for it, like Mr. but would it carry the same connotation as the source material? I cringe when I buy translated versions of Japanese literature due to this (which is why I stick to the source material), it just… does not sit well, I mean instead of writing -sensei, -senpai, or -sama there are “equivalents” in English for those but the catch is that would it work well upon translation?
At a guess, I’d say they retain the honorifics to preserve the culture as it is recorded in the books. It might not seem necessary, but that’s probably because most of us don’t use honorifics any more. We read foreign literature partly to see the culture of the setting.
And it’s not even unique to Japanese. Some languages that are way closer to English have honorifics that are very difficult to translate.
Spanish has a formal (usted) and informal you (tu). And its usage varies wildly across dialects, in European Spanish it’s barely used because it feels too uptight and archaic. Not even teachers are addressed this way nowadays. Meanwhile, in some places in Latin America it’s not uncommon to use the formal talking with your mother. And there’s also Argentina/Chile/Uruguay which has a whole different system.
English had a similar system in old times (you/thou). But it later shifted to exclusively using the formal “you”.