Because they don't have the licence for it. Look at Cote d'Azur - it's not actually the Monaco circuit, but merely based on it. Especially the appearance, but there are a few very minor changes that have been made, most notably in the second half of the Swimming Pool Complex and the hill up to Casino. It's also known as Cote d'Azur rather than Monaco GP because of said licencing issue.
As has been said, real-life circuits cannot be re-created in the game so I'm going to close this thread.