Research software engineering is a hybrid field and fixed roles do not work well. It requires one having a grasp (at some level) of a lot of different stuff, eg software engineerin, the relevant scientific theories, statistics _and_, quite importantly, the culture of scientific practices in a field, in order to make something good. So it gathers a lot of people who, no matter where they started from, often have to sort of converge by learning stuff outside their own discipline.

The problem is not that "physicists write code". Scientists end up learning a lot of stuff and getting good at it (software engineers the same). The problem is that writing software is often left as a job for the occasional phd, postdoc or research assistant, ie people with temporary positions, and in general to people who see building software as a side duty at best, annoyance at worst. This results in no generational knowledge building, being hard to find mentors, less learning and reflecting on practices on how to build software, rediscovering the wheel constantly, too much effort put into . It is not that one graduates as "software engineer" or as a "scientist" and then they know the best practices and everything of their respective fields. People get to learn stuff. Software engineering practices should be in the culture of scientific software building, not simply carrying them from the software engineering world to science, but adapting them and taking it each own idiosyncrasies as a field.