Old habits and a lot of ideas on how social divisions should be enforced in language. With a lot of my friends that work in hospitality jobs, they professionally have their first names visible (and with my spouse and a few others I know, they're not allowed to give their surname to customers), but I guess everyone is allowed to get familiar with them...
My DS1's teachers are all on a first name basis. It hasn't made his teachers less professional nor is he overly familiar with them or with his leaders at cadets who also always use first names even when getting them to march around in uniform. In the case of the same name (which would happen for them even if they went by surname), they'll add on job role rather than title, but only when needing to separate the two.
My DDs teachers are all [title] [surname], and even after 3 years of DD1 attending, some still call me Mrs. when I always sign off emails with my full name with no title and in their computer system we had to sign up to that requires a title, I'm Ms. In fact, this caused an issue when doing virtual parents evening because they'd input more than a few of us with the wrong title (or only put in one parent when the parents evening thing allowed two parents to be put in) so when we tried to sign up, we got errors that parents were unrecognized. Now they're learning all the first names anyways through Teams...
I do work involving the Trust the school is in so I get some emails from the school in the name I introduce myself with when it comes to my being seen as a professional (which I would think of as basic respect) and when I'm seen as a parent, I get emails with a mangle of titles depending on who is emailing.
I do find Sir and Miss/Madam (both of which have connotations that don't line up well with Sir) awkward in schools and really annoying when I have to remind my DDs to use names as I can't know which teacher they're quoting at me when they use Sir or Miss.