Studios because they are investing a great deal of money in movies they want a guarantee that when they hire somebody that person can deliver for them. Everything is fear based so they pigeonhole people. But I've written everything from Westerns to sci-fi to dramedy I've done it all.
The historian must have some conception of how men who are not historians behave. Otherwise he will move in a world of the dead. He can only gain that conception through personal experience and he can only use his personal experiences when he is a genius.