All of the humanities and social science professors or associate professors or whatever that I know pretty much study one thing exclusively. At least, their writing seems to be about the same things. They always say, "This is what I study." And that seems to be about it. Is this true for professors working at most schools? Or if I headed to some Ivy League schools would I find some who are the experts in multiple areas? Or are they all really just so passionate about their topic that they can study it for so long? I couldn't stay on a narrow topic for more than 6 months without needing a good break, and wanting to study something fresh. Even if I found a really interesting topic, I'd still need to move on eventually. Are many able to hop around?
They are passionate about their topic and those immediately adjacent to it such that it affects the original. Professors aren't hired to be jacks of all trades. Then you're not cut out to be a TT professor at a major school. Research studies can take years before they're even ready for publication and the publication process with review and revision can take years before something sees print in a major journal. That said, there are some who once they receive tenure go off on sabbatical to do other things. But you'd need to get to the tenure first, which doesn't happen without significant publications.
Once upon a time I met a man named Louis Loss who taught at Harvard Law School. He was probably about 80 when I met him. He told me that he had once written a law book (it was about the laws related to trading securities). He said that it was very well received and as a result was used in many law schools. He enjoyed writing it but he said that he spent his whole career keeping up with changes and writing new editions of the book. I think he was very proud of his work but there were times when he would have liked to let it go.
