This implies a common assumption among people: That any civilization out there is automatically going to be more advanced than we are.
If you believe that the universe started with the Big Bang, then (IMO), it's reasonable to believe that we all had pretty much the same amount of time to evolve and develop civilizations. Asteroid events notwithstanding.
To me, believing that all beings average about the same place in the development cycle, it makes sense to look for the signals we know, rather than the signals we don't know.
But I'm no scientician.