Imagine an MD who practices medicine for his entire career and is called "Doctor". I can't imagine him retiring and telling everyone to suddenly start calling him "Mister". To date, that is NOT the custom for an RN, NP or PA. Fair or unfair, the MD degree is taken to confer the title and status of "Doctor" for life, even for retired MDs who haven't renewed their license to practice in decades.
No, I'm not in the medical profession as several members of this forum are, but I was raised with a father and uncle who were both MDs, my stepmother, and a couple of step sisters are RNs, and most of my parents friends were medical associates.
Nowadays, many of those people are elderly or have passed on, and my father is 97. Of course he is long retired. Yet all those surviving associates who do not know him well enough to call him by his first name, still call him "Doctor". That's just our culture I guess.
As for myself, hundreds of my former high school students, many now fully adults, some with grown kids still call me "Mr. F." just like when they were in my class. Go figure!