I guess we all misunderstood you then lol. “They/them” is in use because English doesn’t have a gender-neutral singular pronoun. If we did, we would use it. Since we don’t, we use the plural gender-neutral term (and we have always done this when the gender of the singular subject is unknown, too, not just for non-binary people).

This, as others have tried to tell you, has nothing to do with a language having a formal versus non-formal word for “you” (which typically comes in both singular and plural forms).