Liberalization of knowledge

Why is it that only people who have to medical school can be doctors?
If the passed the same rigorous examination even though they have not had formal training shouldn't they be allowed to practise. The examination already tested their knowledge and clinical skills. However why is it that the exams is limited to only a few? The more people in a field the richer it will be and with that amount of competition. Prices will be driven down and quality will go up as doctors try to outdo each other. In the old days that is how it was done. There were medical schools to speak of so people are apprenticed to a doctor help the person do all the menial tasks in exchange for knowledge and skills and when they are ready off they will go to the Royal Colleges and take their fellowship exams. I believe the current and this old system should be used as both as it strenghts and weaknesses but since they are there why not use them all?