The source of most hacking is either through database leaks from other sites where people have used the same passwords or through malware and malicious applications. That means the hackers will have compromised a large number of accounts at the same time. If they try and make a large number of withdrawals simultaneously there is a high chance they will be spotted and blocked by us. If they try to gamble up the balance on each account first they will have a much smaller number of larger withdrawals to make and greatly increase their chance of going unnoticed.