My favorite example of why the government shouldn't be allowed to keep that many secrets is the Supreme Court case which established the concept of "National Security" (secrets). It was a wrongful death lawsuit regarding a plane crash just after WWII (if I recall correctly). The government said, "we can't release any details of the crash because it's a matter of National Security". SCOTUS said fine and the families of the men killed in the crash were told to pound sand.

Jump forward to the modern day, when the details of the crash are accidentally released with a bunch of other declassified information. It turns out that the flight had absolutely nothing to do with "National Security", it was just a routine flight in a plane with a poor maintenance history. The government was just covering their asses to avoid paying out for wrongful deaths, and bullshit so hard they created a legal precedent.