It's part of the arrogance of the government and their power to manipulate public opinion that they've been using for decades. They are smart and we are dumb. That justifies keeping information out of the hands of us, the public.
If anybody remembers the era of the Vietnam War, there were multiple examples of the purposes for which government secrecy was used. For one thing, the Pentagon did not want the news media to be able to show images of the bodies of soldiers who'd been killed as they arrived back on U.S. soil. Why? Because when people are more vividly shown the cost of war, in the form of the bodies of dead soldiers, they are more apt to oppose war.
Also in the Vietnam era, the so-called Pentagon Papers were leaked by Daniel Ellsberg. In 1996, twenty-five years after that leak occurred, the New York Times wrote that the Pentagon Papers
demonstrated, among other things, that the Johnson Administration had systematically lied, not only to the public but also to Congress, about a subject of transcendent national interest and significance.*
Wikipedia says,
The papers revealed that the U.S. had deliberately expanded its war with carpet bombing of Cambodia and Laos, coastal raids on North Vietnam, and Marine Corps attacks, none of which had been reported by media in the US . . . .
But the most damaging revelations in the papers revealed that four administrations, from Truman to Johnson, had misled the public regarding their intentions.**
Copyright (c) 2010 by Richard Stein
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* Apple, R.W. (1996-06-23). "Pentagon Papers". New York Times. Quoted in Wikipedia s.v. Pentagon Papers.
** Wikipedia, s.v. Pentagon Papers.