They evade the common realities of the societies they write about. In the case of novels about girls or women, authors want to give their heroines freer choices than their cultures would in fact have offered. To do that, they set aside the social mores of the past as though they were minor afflictions, small obstacles, easy — and painless — for an independent mind to overcome.The smug presentist science I noted is but one facet of this general tendency, and I think MacLeod gets it right when she brings up the "independent mind." It's this idea that if you're just spunky enough you can Set Yourself Free from your historical context. The desire for a protagonist who is Set Free in this way, as revealed by the way such protagonists appear over and over, is effectively a desire for the opposite of historical fiction. As MacLeod puts it,
Most people are, by definition, not exceptional. Historical fiction writers who want their protagonists to reflect twentieth-century ideologies, however, end by making them exceptions to their cultures, so that in many a historical novel the reader learns nearly nothing — or at least nothing sympathetic — of how the people of a past society saw their world. Characters are divided into right — those who believe as we do — and wrong; that is, those who believe something that we now disavow. Such stories suggest that people of another time either did understand or should have understood the world as we do now, an outlook that quickly devolves into the belief that people are the same everywhere and in every time, draining human history of its nuance and variety.