while one can have overlapping knowledge of both, I think those are, at their respective limits, different skill sets, with different tools and purposes.

but I also think it's mostly industry that really sets people in that way. we end up specializing in one over the other to fit a career path, either by design or by circumstance.

I've known some frontend who were mostly designers who knew/learned how to code in one tool or another, but did not have the deeper understanding necessary to do backend. and I would not trust any of the backend people I know to create a good UX/UI, even though they certainly have the knowledge to figure out how to code it.