Why are so many History books at major booksellers written by non historians? What is their motivation for deciding to write something outside their field? Amateur/armchair military historians don’t bother me too much but I see tons of biographies and political histories by people who don’t seem to have the credentials or any reason to venture into proper history writing.
For better or worse it does seem like a large number of history books are written by non-historians. I think the attraction might be that they write and can relate to the 'everyman' better than the academic. Definitely the high level of thought that goes into history books by historians shouldn't be dismissed. For instance, I love the micro detail V.D. Hanson or John Keegan go into. Though I do find many historians are prone to a structuralist approach of telling history where the main figures matter only and the common are a nebulous group pulled in by various cult of personality forces or religion. And it is the recitations of dates, which can get boring. It doesn't even attempt to put the human element of what drove regular people to do the things that they did. The non-hostorian also might look at history slightly differently. For instance, I read three books on Hannibal back to back. Two were written by academics but the last, and my favorite, was written by a CIA spook. See Ghosts of Cannae by O'Connel He wrote about the psychology of people. For instance, Hannibal's father drove his army out of Carthage and marched it across desert to Spain. He could've sailed them over but didn't. The academics would point to more mundane reasons for this march or perhaps find fault in the plan. O'Connell however thought this was a novel idea because he felt Hannibal's father had only one drive which was to see Rome fall. And that required him to create a preeminent force to take on Rome. By marching his forces across harsh terrain, just as Hannibal did later on in the Alps, it was about weeding out the fair-weather soldiers and leaving behind the toughest most ferocious ones which would be able to take on Rome. It is little things like that that I think the academic might miss in their telling of history, if that makes sense.
If you look at the background of the average non-historian historical writer, an extremely large proportion are journalists.
This is because journalists must be fluent writers and usually acquire a broad (if shallow) knowledge of recent politics, economics, and significant events within a year or two of starting. Then if they're any good, they start trying to build their understanding of causation for those contemporary events so that they can pass judgement and perform analysis better. This inevitably leads to them picking up quite a few history books and reading around a subject from several angles. From there, if you're looking for an alternative income stream (as many journalists do), or if you're compelled to write (as many natural writers are), it's a natural progression to take something which struck you as interesting and run with it.
They don't have the time or training to mine archives however, so instead they collect several dozen related books and read around a subject. That done, they then draw on their (usually humanities based) undergraduate degree skills of extracting the information, synthesizing it into a narrative, and applying their writing skills to try to make it entertaining/gripping.
Naturally, the result varies depending on the capabilities of the writer. A good grasp of detail and excellent writing skills can often enable them to communicate better the research of proper historians to the public (some historians are awful bores and dry reading). Alternatively, some hacks just throw together a poorly researched copy-paste job. The result is really quite variable.
If your question is, why don't academics write more best-selling history books, the answer is: because academia largely disincentivizes work on that kind of book. The kind of history book that is best-selling is usually centered around storytelling; the kind of history book that is rewarded by academics (e.g. tenure, grants, prestige, etc.) is centered around argument.
These pressures exist at multiple levels but they mean that academic history is usually not as "readable" by non-experts as history written by journalists or others used to writing for mass audiences.
There are some exceptions to this, to be sure, and there are academics who have managed to "cross-over" in key ways. But the root of the issue is systemic; if you have a system that not only doesn't reward, but actively punishes academics who write popular works of history, it is not too surprising that there is a lot less of that than you might expect.