Because the writers of the movie said it was.
While the writer's personal opinions are interesting, unless it appears in the movie (and it clearly didn't) their opinion on a common original universe makes little difference.
Canon is what is seen and heard, not what was "intended."
Or, just maybe, the writers thought we were smart enough to figure that out on our own instead of having to spoon-feed us that information in the movie as if we were a bunch of idiotic morons.
You ever see "Blade Runner?" Well, guess what? Deckard's a replicant. Except the film never actually came out and said this. You kinda havta figure that out on your own, because Ridley Scott also didn't think his audience were idiots.