That would increase competition and thus depress wages for existing doctors, who are the ones who make the decisions here. I heard, from a medical school attendee, that she overheard some doctors discussing whether it would be a good idea to require a fifth year of medical school to become a general practitioner (luckily, they were like, "Eh... nah"). It did not seem like it bothered them that this would make it even harder for civilians to get medical care.