More advanced civilizations could be watching out for and preventing more primitive civilizations from polluting the universe with self-replicating probes. We are probably being watched right now and if we attempted to launch an armada of replicating probes they would most likely subtly sabotage them to prevent them from replicating. It would take years before we knew something was wrong and hopefully by then we would be smart enough to not do something like that again. In fact I hope we are smart enough RIGHT NOW to realize that it is highly irresponsible to send out self replicating probes considering how easy it would be for gamma and cosmic rays to corrupt the programming into a virus.