Or it could be any number of things other than nobody out there. For example, advanced life could be out there but it might be rare enough that you only end up with a handful of space capable civilizations at any one time. The milky way is a big place. If we assume that ultimately there isn't a clever way around the speed of light then two civilizations on the opposite side of the galaxy will never encounter each other.
Or maybe its rare enough that you are only likely to have at a time in a galaxy.
Or maybe the prerequisite tech to thrive on an interstellar level means they simply aren't detectable by out primitive sensors. Poor backwards humans, still using the electromagnetic spectrum to measure things!
Hell, we could have a Prime Directive situation on our hands too. Although I think that one is really really unlikely.