Because they are real but unknown. They are unexplained UFO's. Is this what they all said? Are they scientists? Are they familiar with all sorts of atmospheric disturbances, and phenomena? Again, as I have said many times, there is absolutely no sense, rhyme nor reason, as to why Aliens continue to flitter in and flitter out again, with making some official contact, or landing on the Sydney Opera House forecourt, or Westminster, or the White House lawns. This isn't a case or a comparison of us to ants on an ant hill. To any advanced civilisation, it would be obvious that we are "somewhat" intelligent...afterall, we have mastered heavier then air flight, and NEO, and trips to the Moon. And again while I firmly and positively believe we are not alone, as yet we do not have the required evidence to support any thought of life off this fart arse little blue orb. Oh, yes, and I have seen my own UFO, and that's what it remains, a UFO, unidentified.