Are you my identical twin?
I regularly talk to ship masters as a part of my job. I always refer to them as captain. It's a title they keep for life, i.e. even after they come ashore.
And then there is the inverted snobbery that makes surgeons "Mr" regardless of their qualifications! I think it may be dropping out of use, but it goes back to the days when Physicians were University educated, but surgeons were hairy-knuckled types who could give you a shave when they weren't chopping your leg off! I also knew at least one senior academic who was proudly "Mr" - he never earned a doctorate and despite several honorary ones, he refused to use the title!
As you say, few medical doctors actually have doctorates! But you could argue that the length of their training is equivalent to that of a PhD student - 3 years bachelor's degree, then 3 years clinical training.It balances the convention that (medical) doctors are customarily given the honorary title of, er, 'doctor', even though they (or at least most of them) do not hold an academic doctorate.
My favourite tale on related matters is an organisation I worked at, mainly staffed by highly qualified professionals and scientists (several with doctorates) in a range of specialist fields, Some staff put their 'letters' (degrees, professional memberships/fellowships, etc) after their names (or before in the case of Drs.) on their business cards, letters, emails, etc., and some did not. (There are arguments and preferences both ways.) This had never previously been an issue. The Chief Executive then pronounced that henceforth staff were not allowed to do so, for reasons that were unclear beyond 'consistency'. He did, though, insist that he himself continued to be referred to as 'Dr.X' rather than 'Mr. X'.
Fortunately I am both skipper and owner!
Their training has a different goal, so they are not directly comparable. Medical training aims to make someone competent to make potentially life and death decisions, or at least, to know when they need to ask for more expert advice. A PhD student receives training in how to conduct research and how to present and document it. Also, these days the "originality" aspect tends to be downplayed; after all, the expected results will have been forecast when the lead researcher applied for a grant!
I salute you, but you are in a tiny minority!
I remember a plonka introducing himself to me as a Master Mariner, back in the day.
