It is obvious to some professional philosophers, especially the ones who publish in the top analytic philosophy journals, but there is a whole bunch of professional philosophers who primarily work historically. I'd say that at least half of my colleagues work primarily historically. They really just write papers about what
another philosopher said or thought (usually long dead ones).
My impression is that it's primarily a way of immunizing yourself against critique and avoiding academic conflicts. If you write a solid, exegetically sound historical paper you can publish it almost for sure in some journal. No reviewer will be offended by it. You won't make it into journals that require real originality such as the Journal of Philosophy, but you can easily build a career by being a "philosopher X" scholar. Publishing original content and making a name for yourself as a defender of a new philosophical thesis or tradition is much harder.
One of my colleagues works among the world's foremost Wittgenstein scholars and he told me ten years ago that it bores him to death. Nothing has changed since then, however, he's still working almost solely on Wittgenstein.