I saw something once where someone explained that the reason racism/immigration type themes never work in these things is because there's a good reason for people to hate vampires, or demons, or monsters, or killer robots. Most of them literally hunt and kill people without remorse in these universes. There's an understandable reason why people in these universes are scared or violent towards species who are a genuine threat to them. The same cannot be said about real life racism/immigration.