The UFO's are actually AI drones sent from other civilizations that have already crossed over "the singularity." There are no biological entities controlling the craft and the great distance of space makes it impractical to visit other planets. The AI is hyper-intelligent, and we are completely incapable of understanding it (imagine trying to explain algebra to a cat... no matter what language you use, the cat's brain is just not able to comprehend.) Therefore the AI does not attempt to contact us at all (it is not interested in communication, because it can decipher what it needs to know about us without interaction.)
The government is not trying to cover anything up. The (more frightening truth) is that the government does not know what these craft are, or what they want (if anything.) There is nothing to cover up except the mystery of who/what is visiting us. The UFO files are a thousand times more alarming when you realize that no one has come to a conclusion about the phenomenon. It is a chilling idea that with all the evidence compiled about the visits, nothing has made any sense! Look up any unresolved mystery in the past (the Lead Masks Case, the Dyatlov Pass incident, etc.) and you might feel the same kind of unease and horror that government investigators feel. We might have our leads, and guesses, and hypotheses, but we don't have an answer, and that isn't something people want to hear.
When was the last time a politician or someone in a position of authority told you they simply did NOT know something, and it was scary? Sure, police detectives can say they don't know who the murderer was... Scientists can say they don't know how things work... but the government must always say something definitive, or risk losing their power stance.
"Mr. President, what do these visitors want?"
"We don't know what they want, but they are here, they cannot be stopped, and we are unable to observe, catch, communicate, or understand them. We cannot confirm what they are, where they are from, or if life exists elsewhere in the universe -biologically speaking or otherwise."
"Gee, thank you, Mr. President. I sure am glad the currently most powerful nation on earth has things under control! I can finally sleep at night."