I don't think that casinos are willing to backed exhibitions fight,
a. they don't know if it is going to be a hit or miss, casinos are in the business of making money, they have the house edge why venture on business that they might lose in the end.
b. Usually this exhibitions matches are being handled by the boxers themselves, like Jake Paul, he has a promotional company named MVP and then partnered with Netflix to produce the fight like against Tyson. Same with Floyd, he has his own boxing company and so he negotiate his own fight. And that's why he was able to switch referee in the middle of the fight against Gotti III. Imagine that, it hasn't been done before in exhibition or professional fights.
c. Casino is already making money from this fights as they have listed this in their platform whether online or offline. The only thing that casinos can do is sponsored fighters like Adensanya by Stake or some football clubs by Sportsbet.io.