Teaching and research should be decoupled. Professors are hired and granted tenure primarily based on their ability to produce original research. The skillsets are different; often good researchers are bad teachers, and good teachers are bad researchers.
This used to be the case: research was conducted mostly at academic institutions that did not provide degrees [1]. The "research university" is a relatively new thing
[1] https://asteriskmag.com/issues/10/the-origin-of-the-research...
That's not entirely the case in Germany. Applicants need to give a lecture which is public. Usually members of the student union will be present and will have a say later within the hiring committee about the quality of teaching.
But I do agree that the ability to produce and procure research is not at all coupled with the ability to teach.
Absolutely not. You could argue this for entry level lectures, but not at the PhD level. PhD is learning how to do original research, how could you separate teaching that from doing that?
No, they’re usually rated on the ability to bring in grant money.
In this sort of case the “teaching” happening with graduate assistants is teaching how to do research. That’s inextricably part of the job of a research professor, is to teach others how to do the job.
You probably need to step outside of your US-centric bubble if you are to comment on how university works outside of the US. There was a fairly large clue in the parent comment.
There is a case to be made that teaching improves the understanding and insight of the teacher which in turn can increase their research ability. For starters, it provides a less boring way of drilling fundamentals. But more importantly, having to answer questions from students which very likely will be coming from odd and unexpected directions, helps the teacher clarify their thinking. It could well be that one of these odd questions, the answer for which the teacher takes for granted, may actually hold some insight or raise questions into what they are working on outside of class.
In a similar vein, it is recommended that if you are in a business meeting you hear what the junior positions have to say about something first and work your way up the chain of command rather than the other way around due to the junior positions being less familiar with internal processes and thus more likely to flag or suggest something completely out of left field that the higher ups might miss.