1) The most important reason in my opinion is honesty. There are plenty of crypto casinos that operate on an open blockchain and easily provide their winning probabilities, which are involved inside their games, such as roulette, slot machines and the like. Shouldn't every casino be open? Casinos on NON-blockchain do not think so.
I am not sure I fully understand your point. Could you please provide some examples of popular casinos that operate on an open blockchain? It seems like you might be confusing the provably fair concept, which has nothing to do with blockchain technology except for its use of cryptography.