Because they never want to have to deal with the potential problem ever again. They want to move forward and creating a machine moving forward with all the duck

Because they never want to have to deal with the potential problem ever again. They want to move forward and creating a machine moving forward with all the ducks in a row is better than constantly re-evaluating the potential fraudsters -- however small that potential may be. Remember, in this case, Google is the customer and the content provider is the vendor. Google has decided not to be those vendor's customer anymore.