A real head-scratching conundrum about the universe is explaining why it's not already overrun with self-replicating robots. Because if it's possible to send self-replicating interstellar probes, all it takes is one launch, plus a few million years, to get the galaxy overrun with them. So are they not possible? nobody's launched one yet? here, but not detected? The implications boggle the mind.
It may just take them a *long* time to reach every planet. They also may, for example, have a strategy of not visiting every planet as often as possible so as to conserve fuel. They may only visit a planet when it develops detectable signs of life, knowing they command sufficient resources to utterly destroy the existing life at that point regardless of the technological sophistication of the planet. Kind of like if the rest of the world were to decide to declare war on Molokini.