It becomes even less likely by orders of magnitude when you consider the timescales involved. Humans as a species have only existed for a few hundred thousand years on a planet that is 4.5 billion years old. And written human history only begins about 6 thousand years ago. Even if aliens somehow came to earth, what are the chances they would land during the .00001% of its existence when humanity had the capability of understanding what they saw and could document it properly?
The timescales can work in the other direction as well - a self replicating Von Neumann probe could have a presence in every star system within a million years or so, even while traveling at sublight speeds.
It becomes even less likely by orders of magnitude when you consider the timescales involved. Humans as a species have only existed for a few hundred thousand years on a planet that is 4.5 billion years old. And written human history only begins about 6 thousand years ago. Even if aliens somehow came to earth, what are the chances they would land during the .00001% of its existence when humanity had the capability of understanding what they saw and could document it properly?
The timescales can work in the other direction as well - a self replicating Von Neumann probe could have a presence in every star system within a million years or so, even while traveling at sublight speeds.