Remarkable that so many devs spend their entire lives only knowing front end or back end development, rather than both. Why is that? 
Discussion
Lack of curiosity
All hail Gilfoyle.
I'm sure plenty of backend devs have dabbled a bit in react or vue or something, but it requires a completely different mindset to create appealing user interfaces than it does to create functional backends. Specialisation can be a good thing.
Fair
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.
Keen observations