Why would the government hide centuries of evidence of UFO's and alien encounters and still keep it locked away from society?

There's been first hand encounters by so many from military people and astronaughts and even everyday people what does the government not want society and general public knowing about what's really going on that they increasingly want to hide them and classify them
Why would the government hide centuries of evidence of UFO's and alien encounters and still keep it locked away from society?
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