Even though this is being ridiculed, I get it. We typically use a different register (encompassing vocabulary, grammar and accent) for things set in the past. It's a signal to the audience that what you're watching is not modern, just the same way as the costumes are. Unlike costumes, which can be reproduced very accurately, it's not practical to reconstruct the language used in the past so accurately; it will be a foreign language for everyone which is inconvenient, everyone would have to be dialect-coached, and it would make writing the script an absolute ballache. So we compromise.

Still, the juxtaposition of modern language with old-fashioned costumes is jarring to me and I guess many others.