They return their funds because they are traceable, in the case of Casinos they are quick to find vulnerabilities and big casinos are good at doing that, that is why I prefer and highly recommended playing on big casinos they can do refund they have top-level security when it comes to casino security funding and financial capability is a big factor.
Well, you may be right that big casinos are more capable of detecting and tracing where their vulnerabilities being exposed and attacked on because of their higher financial capacity to acquire resources for such. However, it still doesn’t change the fact and probability that they may still get hacked, because as have been stated by the others, nothing on this world can never be hacked. I guess it is safe to a admit that vulnerabilities still do exists in every platform even in this modern era.
The probability is very small because big casinos are very much aware of how these hackers work so they make it a point to hire the best security people in the business, you cannot trust a casino that is a victim of hacking, gamblers demand top-level security on top casinos, and if they are hacked they make it a point to go after these hackers, they have a legal team because they are well funded, so don't get to be a big dog if you know how to retaliate.