This is one of those things that's a bit odd.
It's an artefact of the monetization models.
Doctors have a guaranteed, highly predictable client base with a strongly validated proof of concept in their field. Also, demand for health services is more inelastic (less sensitive to prices) than R&D activities because of the universality and the perceived immediacy of need for what they're offering.
PhD researchers are developing the PoC and generally must make a case for financing their activities to serve a largely undefined client base. Also, because their activities are more detached from the every day experience of most of the population (especially in early stage development), it's more politically challenging to build support from non-traditional sources.
Differences in the incidence of tuition fees across MDs vs PhDs is largely about compensating PhDs on a risk-adjusted basis. The stream of income for most PhD researchers is more variable than for MDs over time and across disciplines. So, high risk equals high reward, in a nutshell.
