Real currency is backed by assets. that used to be the "gold standard", but has become more ephemeral since the end of the first world war.
A government issued currency is backed by that government's infrastructure, taxes, tariffs, etc... basically how powerful that government is on the world stage.
in contrast, crypto is backed by nothing more than how persuasive the creator is because the creator doesn't need any assets to create a crypto currency in the first place.
Heck, in one case, some techbro created a crypto currency, and convinced a bunch of people that it would be stable because he was backing it with ANOTHER crypto currency he literally created for that only purpose.
And people FELL FOR IT!
When something can be created out of thin air with no assets needed but a GPU, it's inherently worthless.