Sadly the reality is even worse than that, everything can be hacked, it does not matter how complex is the protection of a piece of software or a website, hackers can always find a way to hack it with enough time, this is why opening up a casino is such an expensive business as not only you need a lot of capital to begin with but a great deal of the money will always go towards securing your website, because the moment you stop investing in this is when hackers will have all the time in the world to exploit the casino.
^ The only solution for this is don't leave any amount of money of gambling casino, if you don't want to gamble in that day, just pull out all of them or else, never store your valuable assets in the casino. Especially those casinos that have been hacked before, there is a possibility that they will hack again due to their lack of securities. That is exactly the truth, when it comes to the internet, everything could be vulnerable and no one is exempted regarding that matter.
While this is the correct thing to do you will be surprised by how cheap some people can be, because they do not want to pay the fees needed to get their coins out and then the fees necessary to get their money into the casino again, as such they decide to leave their money in the casino thinking nothing bad can happen, but unfortunately hackers are everywhere so casinos and exchanges are a big target for them, which is why despite the best efforts of casinos they still get hacked from time to time and some clients get affected by it.