Bazile........many , many theories BUT:
Back in the 18th century, if you took a University examination then you called be called a professional and all the "others" formed into a Guild.
Now surgeons didn't take an examination, they sort of " apprenticed " themselves to a surgeon and when he thought that they were "ready" they were appointed surgeons to the hospital.
However our surgeons had to be put into some "pigeon hole" so they were joined with the Barber's guild and it was known as the |Barber's and Surgeons Guild.
BUT when the Royal college of surgeons was formed, admission was by examination which made them "proper doctors" but the surgeons decided that they were better than "proper doctors and decided to remain Mrs.