^ Are they, though, or are we just so used to seeing them freely transposed that we've come to accept it as natural?
Well, it's not as if Shakespeare himself didn't update the settings of his own plays. "The clock hath stricken three?" They didn't have clocks that struck the hour in Ancient Rome! His portrayals of historical times and places were generally more like the world he and his audience lived in.
Also a good point. And I fully acknowledge that even his historical plays were (as far as I know) performed in contemporary clothing.
I was just talking to my wife about this tiny controversy. She's a very big fan of the costume drama (and has created period costumes for local theater). Her take is "There are so few movies set in that time period as it is, why pass up a good excuse to wear all those neat clothes?"