It's low because there's no competition. MDs are like medieval guild: once you're in, you're set for life. Restrictive regulations are lobbied by MD associations, which limit competition.
Q: What do you call the worst student to be admitted to medical school?
A: Doctor.
The attrition for lawyers is higher, neither has anything on PhDs where the number of "practicing" doctors drops off constantly during and after the degree, and only really levels off when O(10%) of them land faculty jobs.
[1]: https://www.ama-assn.org/medical-students/preparing-residenc...