It is real because the people who use it believe it is real. Same goes for zero-dividend stocks, crypto, Yu-Gi-Oh cards, Beanie Babies, etc. The difference between currency and any ofbthese others is only in the number of people involved.
You can point to government regulations for money. You can point to SEC regulations for stocks and other securities. I can point to algorithmic scarcity for crypto. And I am sure there are standards that various collector communities deem important. But, the fundamental concept value for any of these others is that the people using it believe it has value.