The reasons for this are largely logistical and academic:
* Rigor and Depth: Doctoral programs require a significantly higher level of independent research and scholarly contribution than Master's programs. A combined program would struggle to adequately cover the depth and breadth needed for a doctorate in the allotted time.
* Accreditation: Accreditation bodies generally require distinct programs for Master's and Doctoral degrees to ensure rigorous standards are met for each.
* Curriculum Design: The curriculum for a doctorate is fundamentally different from a Master's. It's difficult to structure a single program that seamlessly integrates the different learning objectives and research requirements.
* Research Focus: Doctoral studies emphasize original research and a dissertation. Integrating this intensive research process into a shorter, combined program would likely compromise the quality of the research.
While you won't find a combined MD/PhD program online, you *might* find universities offering a fast-tracked path where completing a Master's significantly reduces the time needed for a subsequent PhD program. However, these are usually still two distinct programs, even if closely linked. You should carefully review the program details to understand the structure and requirements. Always verify the accreditation of any university or program you consider.