Big banks think they can get away with it
Small banks are viewed as being somewhat vulnerable to money laundering operations due to their size and lack of resources. It costs a lot of money and takes a lot of effort to comply and have the right processes in place. You would think the bigger banks have the economy of scale to incorporate effective AML barriers. This is true, but on the other side of the coin, larger banks don't really see AML compliance as a top priority. Perhaps also, having many more customers and banking reps, AML 'oversights' would be more common and harder to manage. More customers =increased vulnerability criminals.