Not only gambling casinos but also in exchange platforms as well as long a site does have lots of funds involved then they are really that susceptible from hack because they do know that this is the sweetest spot on
finding out vulnerabilities and loop holes which they could really able to steal websites funds specially on hot wallet but we know that not all casinos does store all of funds on a hot wallet but also
into their cold storage to prevent out completely wipe of funds which is very common behavior for platforms like these.
If it were a business on the internet, it would all be vulnerable to hacking. Also, hackers won't be too picky about their targets, and as long as they can get onto the site and get what they want, they'll be sure to stick with it until they're satisfied. That is why gambling sites or exchanges or even online shops need to be aware of attacks from hackers because they can attack at any time and enter at any time when they already have access. We will never know how they did it because many of those hackers leave no trace of them when they enter a site.