I agree with your last point. I suspect MD is a popular option because it has job stability and good pay (which is likely related to the scarcity of MDs in the US).

In my field (another type of research) a lot of people "want" to be professors, but when I talk to them there's no interest in teaching or doing administrative work, they just want job stability. If research jobs were stable they would happily spend the rest of their lives doing research.