It's not the only way. However, I will tell you that doctors usually enter practice around the age of 33, 35 or 36. They have a lot of student debt then, often amounting to a few hundred thousand dollars. They have to set up their practice and start a family. So they can't start thinking about investing for retirement until they are about 40 years old, when all this is reasonably accomplished.
If they invest in an RRSP, for example, at a maximum of $20,000 per year, how can they save enough capital in the next 20 years to provide them with a return and to ensure a decent retirement? They're a little stuck. They would be among the only citizens who had contributed much to society but did not have access to a suitable retirement once they reached that age. This is a problem for most of our doctors.