sure - there are absolutely lousy PMs as well as great PMs, but this kind of situation happens in all kinds of organizations and I think you are overgeneralizing.

There are lots of reasons that excellent scientists might not want to actively do research anymore. Being a PM seems like a nice way to get a broad exposure to cutting edge research without dealing with the (IMO) overwhelming incentive structure in academic science, driving a narrow research program at a national lab, or leaving the world of open science to as an industry researcher.