They aren't. There are numerous reports of incidents of ET contacts and coninciding government coverups, internationally, and we know of them only by the fact that they were not successful in their cover up. My take on ETs visiting Earth is this: For all the hundreds of thousands of 911-calls, sightings, videos, pictures, and reports, what are the odds that they are all hoaxes and weatherballoons and drunken psychos. My theory is that the media and the scientific community, the only entities capable of "certifying" a visitation, are so arrogant and idealogical in their denial that they refuse to even consider the possibility and examine the evidence. Some things are so scary and earth-shattering that we ignore them even while they stare us in the face.
Assembling a large quantity of noise does not guarantee the presence of a signal.