I think you're overall correct, and you don't need to be actively conducting research (in residency or practice) to be a good dermatologist. In fact I think that a resident who does 0 research in dermatology residency after getting in, and instead focuses on studying and getting high volumes of patients will, be the same or better than one who prioritizes research over these other areas. A lot of dermatology residents will just do the minimum requirements set out by their program for research and still go on to be great physcians. But it's near impossible to finish dermatology residency without doing at least a small amount of research. At my medical school's dermatology program, quite literally every single resident was doing multiple posters and case reports with several different medical students, and some residents were even doing more extensive projects as well.
Ultimately, I think it mostly stems from the fact that dermatology is so competitive that they need to create even more hoops to cull the applicant pool, and (in faculty's minds) what better hoop is there to jump through than research, which is ubiquitous in dermatology departments and needs constant manpower. Sure having a research understanding is very important, but the explosion in quantity and importance of research has skyrocketed in the last few years and I don't think will end up producing better dermatologists than 5 or 10 years ago. I'm sure research is just as valued in neurology, pathology, rheumatology as well, but these specialties just can't force their applicants to dedicate 1000+ hours to research prior to applying as dermatology does.
Personally I think being willing and able to study and read constantly, combined with high clinical volume, is the optimal path to training the best dermatologists. Understanding research methods and being able to evaluate literature are critically important in dermatology (and other fields as well), but actually conducting the research yourself isn't going to make or break you as a dermatologist.
tldr; The research requirement is because dermatology is competitive, not because you need to conduct research to be a good dermatologist.