A vast oversimplification, but currencies hold value because of a combination of being backed up by material assets, and widespread trust in the backer of those assets.
If your currency is neither backed by something of value, nor trusted by literally anybody, then your currency holds no value - end of story.
If anybody could just write note on a random piece of paper that said it was worth $10 million and cash it at the bank, don't they think somebody high-profile would've done just that by now?