I believe this is a British thing that the Americans adopted. Last names were the form in which all students were referred to, both by teachers and other students, at least on the playing fields of Eton. And what else counted? The form was continue throughout adulthood.
In the Sherlock Holmes stories, Holmes is always referred to as Holmes, except by his brother, and Watson is always referred to as Watson, except by his wife, who called him James. Yes, that’s true. Look it up. Doyle forget what Watson’s first name was. He was a hack writer of the worst kind. Moriarty was given the first name James, and so was his brother. (We only refer to John H. Watson because it’s on the dispatch box where he keeps his manuscripts.)
In later British mysteries, Hercule Poirot is always referred to as Poirot. Dr. Gideon Fell always by Fell. Gervase Fen by Fen. (Thankfully).
Nero Wolfe is always Wolfe, except for a few close friends. Archie is a servant. Perry Mason is always Mason, except for a few close friends. And the model Robert Parker took from was Richard Stark, a pseudonym of Donald Westlake, who called his no-first-name hero Parker. How could Parker not?