It is not impossible to "hack" a casino... but the more popular casinos have check and balances in place to immediately identify "losses" for the casino..so you will not do this for an extended time. What mostly happen is that players find some kind of loophole in the 3rd party games and they exploit it.
I recently informed Stake.com about a loophole that was being abused by highrollers on the game "Mount Magmas" and they immediately disabled the game for review. (Push Gaming will look into it and then change the code and it will be re-introduced, after it was fixed according to Eddie)
Most of the "house" balance is stored in cold storage... so a hacker will not get to those funds.
It is 100% possible to "hack" a casino. Yes, most big casinos like stake or sportsbetio or whatever have ways to prevent something like this happening. Mainly they have a hot wallet and cold wallet which means the best you could do is hack into their hot wallet and empty that, it is the biggest thing you could do. However what people do not realize is that hacking into gamblers is a lot easier.
You could literally hack into casino and get information regarding the gamblers, like username and passwords, which is out in the open for most hackers who can do it, and then they could test to see which one has 2fa and which one doesn't and whoever doesn't have it will get their account emptied. That seems like a lot easier situation if you ask me. At least easier than trying to hack into the hot wallet and get out with a small amount of money.