there is nothing behind it
There is unbreakable cryptography behind it. Science and math. That is 'harder' currency than any fiat that will ever exist. A truly secure and sound form of money, along with the transparency to shine light on all the corporate corruption in the world.
Now. Tell me again it has no value.
Happy to: It has NO VALUE.
The problem here is that your definition of "value" is invalid. Your claim is equivalent to saying that shares of Apple stock have value because we have built systems to ensure that ownership of them is unique and non-duplicable, and that the supply is finite. But that is not what makes shares of Apple stock valuable. What makes them valuable is that they represent partial ownership of a company that produces and sells hundreds of millions of useful devices every year, among other things, and generates hundreds of billions in profits.
Shares of stock, dollars and bitcoins are all non-duplicable, finite-quantity tokens, but the shares of stock and dollars are tokens that represent something in the real world, while bitcoins represent nothing beyond proof that some electricity and some hardware was diverted to create them and transact them, rather than put to a productive purpose.
Yes, it's true that the guarantees of the non-duplicability and finite quantity are stronger for the coins than the others, which could theoretically make bookkeeping easier and reduce the need for auditing of transactions, but it turns out that the cryptoasset guarantees are hugely more expensive than the old way of bookkeeping and auditing, and the non-reversibility of crypto transactions is a fatal flaw for serious real-world use (though it's a boon for scams). If cryptoassets were easier and cheaper to exchange than traditional methods, and if there were a good way to address the reversibility gap, it would make sense for countries and companies to switch to using crypto-tokens rather than audited database entries. Maybe someday someone will invent a crypto-token design that achieves that, and then we'll trade shares of Apple stock by exchanging tokens on a blockchain (or similar). But even if we do that, it will still be the case that it's the ownership of a productive enterprise that provides value, not the token, no matter how elegant the cryptography.
Note that I'm not saying this because I don't understand the cryptography behind cryptoassets. I'm a mathematician and a professional cryptographic security engineer; this stuff is literally my day job, and has been for most of my 35-year career. I deeply understand the (actually quite simple, though cleverly assembled) cryptography and precisely what it does and doesn't do. It strongly ensures that there is a limited supply and that coins cannot be double-spent, and it does this in a decentralized way. That's it. The limited supply and non-duplicability are properties required of any asset, but aren't what give an asset value.