Aliens have visited earth many times. But so far, it is always in the imaginations of a species desperate for answers and connection to the cosmos.
Absence of evidence is not evidence of absence. Nevertheless, no truly compelling evidence such as a fossil, a body, a true spacecraft or even a credible photo or video exists. And, somewhat like belief in a deity, true believers are not at all persuaded by this. This is because most human beings are wired very deeply for wanting connection, and seeking a set of true answers is part of the human condition.
Modern psychology, and especially ecopsyhology, and Biophilia teach that people are alienated from the planet of their ancestors and natural living systems. People long for something to reconnect to that source. Earth could be thought of as an enormous egg, fertilized by pan-galactic projectiles , (such as comets and meteors), that brought the building blocks of life. Yet, beyond these "aliens" who may have introduced the chemical compounds necessary to form life, no tiny green men are hanging out around here from outer space.
Also part of the human condition is human gullibility and failure to hone skeptical skills. Critical thinking first consists of a person recognizing they “want to believe” and then also recognizing they are very prone to suggestion, association and hope.
Knowing for absolute certainty that there are others out there in the cosmos something like “ourselves” satisfies an amazingly deep hope and need. Once people admit this, they are not so prone to the very real temptation to want to believe.
What about people who are threatened by, or afraid of alien life? In this situation as well, it all comes back to the human need to be special, to belong, to matter. If the aliens are menacing and horrifying it means they at least appreciate earth, or humans, as something worth investigating. Not getting probed every night? Maybe Zax Flax from Centauri Six is just NOT that into you.
That may sound harsh, but it is true. The interstellar distances involved in galactic travel ensure that any species of inter-generational time spans, and advanced technology for it, would have as much in common with earthlings as earthlings have with cyan bacterial stromatolites from ancient earth.
Human beings have created life-forms from the sky from Zeus, to Thor, to angels, to creation ravens and to ghosts and more. Therefore, it is logical to conclude that, (despite my typing of this article from the Mother ship of Glatz Bleep just to assure you we do not exist) extraterrestrials are products of the mind and hearts of homo sapiens hungry for revelation and kinship to something greater than themselves.