BlindersWilliam Rubinstein posts on the difference between academic and amateur history, the former being too focused on minutiae, the latter a number of subjects that have broad interest. At least with respect to academic historians, few historians reach beyond their narrow fields of expertise in order to look at longer trends in history.
Most non-historians, however, do not realize that microscopic analysis of this kind is built into the post-graduate education of would-be academic historians anywhere in the world. Obtaining a doctoral degree (a Ph.D.), the sine qua non for employment as a university lecturer or its equivalent, centrally entails writing a dissertation of up to 100,000 words, based on original research, demonstrating that one is a capable historian at an advanced level. Almost necessarily, a doctoral candidate writes on a very narrow topic: it is impossible to do anything more in the time at hand. In any case, one's examiners regard the mastery of a narrow field, rather than an attempt to do something more ambitious, as evidence of genuine historical ability. Historians' publications normally grow out of their original research; hence the concentration on the narrow. Of course, some academic historians write wider-ranging books, although few would ever wish to venture out of the limited areas where they regard themselves as experts.
Many historians don't go beyond their narrow fields of interest. However, I feel that all graduate students are given the opportunity to broaden their fields of interest. I am unfamiliar with the English university system, but requirements such as comps fields outside of one's main fields, foreign languages, lectures and other events supported by the department, and normal camaraderie -- even teaching assistantships -- are venues in which students can explore and compare work in other fields of history as well as other disciplines. I think that it is unfortunate that students take a narrow approach to their formation, treating requirements as a burden and eschewing the opportunity to learn more -- particularly if the only goal is to get the degree and get on with life.
Full Disclosure: My outside fields were in history of southern Africa and literature and culture of West Africa (done in French).