There's nothing that is "very easy for you" even those people that are working to steal others fund will have hard time to hack those reputable gambling sites. Because most of have tight security and they invested big money for the security. If a gambling site doesn't have a tight security, he doesn't know what he's doing.
This is because gambling sites have invested tons of their profits in securing their website and gambling games. Another thing is that they don't put all their funds in their hot wallets, but they put it in cold wallets that are not connected to their servers. So in case anyone does hack into it then, they would only be able to steal a few coins.