Professional pride. Some professions are not proud enough to call it by the name. An electrical engineer or a mechanic or an architect or a civil engineer is always addressed by name when talking to one, not really Mr Mechanic or Mr Electrician - can you please fix my car ? or insulate an open wire, as it looks very tasty and I might lick it by mistake but when it comes to doctor of any kind, I mean really any kind from foot to head or practicing (this word scares the shit out of anyone I guess - old joke) any type in the lines of English medicine or homeopathy or Ayurvedic is still addressed as Doctor and not by name. If you think this is tied to the title, why a head cook is called chef and not by name? Let’s not even think about American police, they are neither Police nor do they have a name, from nowhere they are officers (most of them we talk to are on field and don’t even have an office). A judge being called a judge when not in court of law sounds pretty rude isn’t it? Judging someone when you have no business with them is considered rude by certain moral people in the society, now you call a judge by the profession while one is having a cup of coffee in a cafe and minding some business totally un related, to judging people is very rude. (God I have used the word judge so much in the last sentence, my phone’s first suggestion is Judge when I type the letter "j", before that it was jam, no prize for guessing why).
After we have created enough confusion and let you ponder over other examples, I think this professional title is related to passion in majority of people. Doctors or chefs are more passionate about their profession than electricians or mechanics or lawyers. I don’t mean all of them, there are exceptions amongst doctors or chefs who do hate their jobs (now that thought scares everyone isn’t it?). Meanwhile there are electricians, mechanics, lawyers, bloggers, vloggers, who like being one but still people call them by name or Mr or Miss or by name of one of the hollywood stars (my personal favourite is "Tom cruise" fixed my car, even though the engine was reachable by just standing on the floor and floor had no weight sensors he just came from the roof hanging to change oil).
It really has to be the passion of profession and not title. If it was the title then it can only justify a doctor being called a doctor, not a chef. What do you think is the real reason for professional titles?