ELI5 : Why do British and Australian people sound like regular Americans in movies?
Do they? Do you mean that British and Australian actors often talk in a american accent in a movies? That is not the same thing.
The is done because the director what the character to talk like that and if a British or Australian actor what to play that part the have to be able to talk in a american accent. It is a good idea for them to learn that because there is a lot more work in the US then in there home countries.
Because they’re playing a role that calls for an American accent. It happens the other way, too, like when Mel Gibson played a Scottish man in Braveheart.