The United States Government certainly does know about the existence of UFO's and aliens. One would be surprised on how much they know. Although they know about them, they will not release the information to the public for several reasons.
First the religion conflict. Major catastrophic incidents would transpire if alien information was released in a civilization that isn't ready to know the truth. People prefer to live in a naive manner denying anything that is unfamiliar. But soon they are in for a huge shock.
The Government knows they are defenseless against Aliens, and our technology and weapons are no match for more advanced extraterrestrial technology. If these aliens ever posed a threat to us, we would have no way to defend ourselves. Just the fact of knowing our future relies upon unknown forces will cause major panic.
The Government likes to be controlling. If they can not control the situation, they will work hard until they can. There job is to protect and serve the people, and until they understand everything, they will not reveal this information.