It’s a bit of reverse snobbery - historically surgeons were literally butchers, as they had the tools necessary to do amputations, which was just about the only operation going in medieval times. The physicians at the time refused to refer to them as doctors.
Now when a surgeons joins the Royal College (requiring extra exams post med-school), they actually drop the title Dr. Being called Mr again is something they’ve had to earn.
Then some of them do PhDs and become Drs again. Fickle bastards.