As an aside why do people call themselves Dr just because they have an academic qualification
Doctor was used for academics well before it was extended to physicians. It etymologically means teach(er).
Doctor fell out of favour with many academics when it become associated with quacks (there was long debate about medical professionals who hadn't earned a doctorate should continue using it then as well, but it was traditional by that point), but in more recent years there has been a push in some areas, particularly towards women and other marginalized groups, for those who've earned a doctorate to use the title more.
No need nor justification to use Sir, Major, Doctor, Reverend or anything like that when doing online shopping or registering for a new dentist. They have no place at all in "normal" life.
There is no need for any title in day-to-day life. It only started to be a thing for everyone to use one in the 19th century when it moved from professions to a class thing. We only use pronounce Miss / Ms / Mrs for women as we do (when we pronounce Mr as Mister or Master - not Mur) because Mistress - along with so many others things for women - was sexualized.