Um...because we are a fearful and ignorant race that can't even get past the differences of skin tone and culture among our co-inhabitants on this planet? Because they would likely either encounter the barrel of a shotgun, or the scalpel of a scientist?
Perhaps they have landed, quietly, in remote locations, to make contact with those few spriritually advanced individuals among us who would be able to accept their reality.
Just a thought.
Which just goes to show you: UFOs are about religion, not science. These are things you either "believe in" or don't, not things to be scientifically proven.
Well, UFOs are simply Unidentified Flying Objects. I have seen things in the night sky that were flying or hovering, which for the life of me I could not identify. So as far as that goes, UFOs absolutely exist.
Scientific history consists of things being discovered that were previously not known to exist. Hypothoses, theories, and hunches were tested, and proven or disproven. Uncharted regions of the earth were explored, new species found. Before DNA, or germs, or the Western Hemisphere was proven to exist, did these things have no reality of their own? Was it a matter for religion and for fools to believe in or not?
BTW, religion and spirituality are two separate things. The first is imposed from without, the second is generated from within.