What evidence do you have that corrupt FBI agents are rare? Just because OUTSIDERS have only publicly caught a few does not suggest those are the only ones.

There is also more to the problem than corrupt individuals. Sometimes incorrect attitudes (racism, mistrust of whistleblowers, unjust patriotism) can result in organizations doing awful and immoral things even if single members are not publicly charged with a crime.

Or when an organization believes it knows better than the constitution (https://en.wikipedia.org/wiki/James_R._Clapper#False_testimo...)