Exactly. You've got limited time to tell your story in a movie, so establishing a fake currency that nobody except hardcore numistatics nerds will care about is kinda pointless when everyone already understands that credits = future money.

And as you say using a real currency doesn't make sense in most future movie contexts, and using the "wrong" one may even offend some people who may then boycott your movie - which is obviously not desirable when you're a studio trying to make money.

I image its the same reason for the Gold/Silver currency in many fantasy settings.

I think in Rick and Morty one of the fake currencies is called a schmeckle, which is Yiddish for penis (diminutive).

No, they won't.

The money can’t be any real earth currency

no, now that you've said this i want to see a sci-fi movie where all trade is based on the Vietnamese Đồng