Apparently this is not quite true. Via April DeConick I read this article by NT scholar Gerd Ludemann on the historical accuracy of Luke-Acts,. Luke-Acts is a prime example of the kind of biblical writing that needs explaining because there is a plain-for-all-to-see theological agenda in this two-part history of the early Christian movement. And there are obvious discrepancies between Luke and the other gospels and between what Acts says about Paul and what Paul says about Paul. Luke clearly changes his telling of the early Christian story to fit his theological agenda.
But didn't everyone do this back then? Here is Ludemann:
A number of Greek and Roman historians engaged in an ongoing discourse about historical truth, including the issues of intellectual deception and falsification. Moreover, those of the Greek and Roman intelligentsia, though lacking the sophistication of modern critics, had developed clear criteria by which to uncover fraud.Plagiarism and pseudepigraphy happened; but they were not considered acceptable by everyone. It was the constant task of librarians, Ludemann points out, to uncover fraud. There were historical writers who tried to pay attention to verifiable facts in a manner akin to the writing of modern history. (And yes, I know, that all history writing has a bias.) There were pre-modern histories that did not take for granted miracles; ancients could be as skeptical as moderns regarding explanations of events that resorted to supernatural intervention. Apparently not everyone was doing it.
But the biblical writers were. I don't know how much difference this makes in the way I approach the teaching of scripture. It still needs to be understood in context and it still needs to be seen as a form of story-telling that takes historical events and weaves them into a story that has a primarily theological and apologetic purpose. But I won't be able to say anymore that everyone else was doing it too.