Particularly for group corruption, everyone is in the same boat, and the participants take care of each other. Unless there is infighting or problems caused by an uneven split of the take, such cases are unlikely to be exposed and attract attention. Hence, it's extremely difficult to get evidence.
尤其是集體貪瀆，大家都在一條船上，都把彼此保護得好好的，除非有窩裡反、或是分贓不均，才有可能爆發出來，因此在取得證據方面非常不容易。