That's carrying a lot of assumptions. That there's not a probe lying somewhere in the solar system watching us, that aliens haven't found a better way to observe us than probes, that they even care about observing us in the first place. Take the last one for example - 400 billion stars in our galaxy alone. It's entirely possible that we're not important enough for advanced societies to care, the way you're not going to stop and mess with every random anthill you pass by.
Imagine a couple of tigers talking. "Someone says there are 9 billion humans in the world." "That makes no sense. Take the amount of good hunting habitats in the world, divide it by the amount of territory an adult male would need to call their own, and divide that by the number of humans. You would have multiple humans fighting for each chunk of territory, and marking their scent everywhere. Yet I don't smell any human urine so the question is...where is everyone?"
As an aside, why do people focus so much on self replicating probes? Assuming aliens did want to send out a probe fleet - why not simply mass produce probes in one system (or a handful of systems) and send each in turn to their destinations? Having to accelerate, decelerate, build a probe, have the new one accelerate, decelerate, build another probe, have this new one accelerate, decelerate, etc., seems like it would take a lot longer than simply sending a probe directly to its destination (no need to constantly decelerate along the way, no need to waste energy moving replication units, etc.).