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/.
It is the brain of human that found blockchain, and so, it'll be human himself who'll find a solution for those fiat casinos to implement this "provably fair" thing if they really want their players to know everything about their bets and to prove that they've not cheated over them (users).
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...Why are Fiat online casinos not adopting the new provably-fair technology?...
I'm going to react because of option (b).
To me it's pretty clear. Why would casino bother implementing something that could negatively affect their profits when it's users don't require such thing.
Once players realize provably fair concept exists it can be game changer but first they need to start complaining (or stop playing). Apparently this is not happening.
In addition casino must be sure the lost profits (due to people stop playing their unfair games) exceeds lost profits due to offering actual fair game. Unless this condition is met, there is no will to change anything.
BTW I fully agree with SyGambler post.
That's true, no casino is stupid enough to strike a hammer on their foot by adding such algorithm as "provably fair" because it'll help a lot of users to validate if their loss was actually real or they were cheated. I tried to find a casino where a player could get a verifier to check if their bet was provably fair, but none was found. Bet365 is one of the biggest names in the gambling industry and has a lot of users playing there all day long and they have a dice game too, but there were many users complaining about their losses as it's really easy for a fiat casino to cheat on its users by not letting the users verify their rolls through any means.