Because they would be so light, nanorobots could easily be catapulted at near light speed to exoplanets, where they could then replicate to build machines which could then catapult them again to other exoplanets. Because the nanorobots would require little resources, this could be done with a large number of such nanorobots so that even if most miss, some would arrive. In this way, the entire galaxy could be explored in less than a hundred millennia. If there is any other civilization anywhere in the galaxy, it must have made its way here by now, even if it does not interfere. On a cosmological timescale, civilizations advance so quickly that it's extremely improbable for two civilizations to be at a similar enough level that contact would be meaningful.