Possibility #1: Some sort of Galactus/Unicron type being that consumes entire planets.
Possibility #2: Physical space is an incubator for higher minds, who upon reaching a certain level leave this realm and thus are not sending out broadcasts of alien I Love Lucy for us to detect.
Possibility #3: Perhaps amidst all the UFO hoaxes and sloppy ancient astronaut theories there is an actual account of an actual visitation.
Possibility #4: While there may be other intelligent life in the universe, there is no particularly strong reason to assume we are not the first and oldest of them.
Possibility #5: Every time an intelligent race gets sophisticated enough, they choke on their own pollution and/or bomb themselves into extinction over subtle ideological differences.
Possibility #6: We are living in a vast computer simulation and, for whatever reason, the simulation only spawned one race of intelligent "life" and it's right here.
Possibility #7: There is intelligent life but they are for some reason (intentionally or not) concealing their presence from us.
Possibility #8: The life that has evolved elsewhere takes such radically different forms and utilizes such wildly different technology that we could be looking right at them and do not even realize it.
Possibility #9: God made Adam and Eve, not Xadam and Ee'hv.