What makes you think that an asset needs to have a country backing it for it to be considered valuable?
Also, there is absolutely no one who says that an asset needs to be physical in order to be valuable. There are plenty of funds in the world that are in virtual fiat currencies, stocks, etc.
Bitcoin's valuable not from a legal standpoint, but rather from the fact that people trust in its decentralisation, and the fact that the supply can't be tampered with by any central entity. The fact that you can transact with it on a global scale without middlemen is what makes it valuable, not because laws state that BTC is the national currency of a country. That's the distinction between bitcoin and fiat currencies.