Well, that's because there's a cryptographic algorithm that makes something provably fair. The data has to be hashed and then 'reopened' to see if it matches the initial one. Blockchain allows such things to be performed, while fiat doesn't use cryptography. The reason that the same same provably fair thing cannot work with fiat is that blockchain records data automatically and nothing can be changed by people after that, while with fiat people have influence during many stages of proceeding a transaction. Read more in this article to understand why fiat provably fair casino doesn't really make sense: https://bitcoinchaser.com/provably-fair-gambling/.