Not a stupid question, Karla. It is kind of an odd phenomenon.
The way I see it, the people who make movies/tv like to use British actors/accents for any setting that is not the US (so, anything from present-day France to 14th century Italy to Middle Earth) because the characters need to sound different from the way we would normally talk. But, at the same time, they know that British accents are easier to understand than, say, French accents, and so they use those.
There's also something called a "stage accent" (I think that's the right term) which isn't technically a British accent but might sound a little bit like one because it sounds more proper, I guess would be the word for it.