I don't mind a bit of "historical escapism" with 21st-Century-style characters dropped into a setting they're unlikely to have developed in, but I certainly share your view that its prevalence is worrying. A bit more balance and historically considerate fiction would be welcome.

It goes back to Walter Scott and the birth of the modern historical novel. The problem is that for a lot of readers, too much of this kind of thing skews their mind-set: the expect people in the past to be 'just like us but in fancy dress', and give howls of outrage when they discover otherwise. They don't seem to realise that history is as much a branch of anthropology as anything else: but the strange and curious peoples one studies are exotic and distant in time, not necessarily geography, and are often one's own ancestors.