There are so many reasons NOT to.
Big reason is, if you deal with fiat, then you need to get licensed, you need to get regulatory approval, you need to prepare your entire system to work with a fiat system.
Do you honestly think you could enjoy low house edge, fast approvals, fast play, and all that, while using fiat systems? The whole reason dice games and crypto games can offer much lower house edge is because of lower operational cost. Choose to use fiat and kiss goodbye to those low operational costs.
Ton of existing Fiat online casinos already exist and they make millions of profits; although they have all the disadvantages that you have mentioned: Need to get licensed, get a Fiat payment system, etc.
But just by changing the random-number-generator of a Fiat casino to provably-fair would give them a huge advantage over traditional online casinos with intransparent algorithms. Also a provably-fair Fiat casino could target a much wider audience than the current coin-based casinos.
I still don´t understand why no large Fiat provably-fair casinos exist yet!?