Even if we pretended aliens wouldn't be able to surpass FTL, Von Neumann probes only take half a million years to spread across an entire galaxy, which is a grain of sand of time in cosmological timescales.
"Let's take a look at one of John von Neumann's most fascinating contributions to science: the Von Neumann probe. Simply put, a Von Neumann probe is a self-replicating device that could, one day, be used to explore every facet of the Milky Way in a relatively small window of time.
The general idea is to build a device out of materials that are readily available and easily accessible out in space, like on rocky planets or small moons. Once it finds a suitable destination, it lands and mines the material it needs to build even more devices, which, in turn, land on other planets and moons and build even more.
The system is very effective, and by some estimates, it would take around half a million years to dispatch millions of probes across our galaxy, assuming each one travels at approximately 1/10th the speed of light, or 18,640 miles (30,000 km) per second (though the real number could be closer to ten million years, which is still no time at all in the grand scheme of things)."
What confidence do we have that we'd actually observe these probes? If one was orbiting, say, Saturn or Jupiter (or one of their moons) right now, would we definitely have seen it? Or if it were something in a wide solar orbit?
And then there's the general time scale. I agree that half a million years (or even 10 million years) isn't much in the grand scheme of things. But it matters to us: what if this alien civilization sent them out 50 thousand years ago? They may not have gotten to us yet. Or what if they were sent out 100 million years ago? It's unclear if they'd still exist today, or if they would have broken down, their repair and replication systems failing after all those years; even if they are still around, maybe the civilization that sent them out doesn't exist anymore.
Also consider that building these probes is pretty much at or beyond the limit of our technological capabilities right now. We might be able to build enough of them as a seed fleet if we devoted tens of trillions of dollars to the project. And accelerating something to 0.1c is nothing to sneeze at either. Remember that they also have to decelerate from that speed in order to find targets for mining in order to fuel themselves and their replication systems, or even just to enter orbit around a planet or star. They'd require some pretty advanced software to run them, too.
I've seen quite a few articles and videos on Von Neumann probes, created by people I respect, but even many of them acknowledge that building these and sending these out would not be a small undertaking. A civilization that did so would have to be at a point where they've solved most of their problems at home (or be in dire need of finding a new home, but at the same time able to wait thousands or millions of years to find one) before they could justify taking this on.
(Remember, the person upthread isn't talking about the mere existence of aliens. They were talking about the improbability of us having some evidence or experience of their existence.)
"Let's take a look at one of John von Neumann's most fascinating contributions to science: the Von Neumann probe. Simply put, a Von Neumann probe is a self-replicating device that could, one day, be used to explore every facet of the Milky Way in a relatively small window of time.
The general idea is to build a device out of materials that are readily available and easily accessible out in space, like on rocky planets or small moons. Once it finds a suitable destination, it lands and mines the material it needs to build even more devices, which, in turn, land on other planets and moons and build even more.
The system is very effective, and by some estimates, it would take around half a million years to dispatch millions of probes across our galaxy, assuming each one travels at approximately 1/10th the speed of light, or 18,640 miles (30,000 km) per second (though the real number could be closer to ten million years, which is still no time at all in the grand scheme of things)."