Earlier quoted context omitted.
Not really unlikely if you consider the vast scale of time as well. A few million years is enough to reach the end of the galaxy even at very very low percent of the speed of light. That's a long time in our terms, but a very short time given the age of the universe, or even our own planet. Self replicating Von Neumann probes could handle the issue of exploring such a large number of solar systems by increasing their…
i think the main issue with people having trouble believing in visitations from extraterrestrial species (or at least their technology), is that they have absolutely no sense for the age of the universe. and the fact that so many people believe it to be utterly ridiculous that alien life forms could be exploring the galaxy, yet here we humans are, doing exactly that, is just completely mind-boggling to me.
The fact that they haven't suggests that either they don't exist or that they haven't found us.