Why do you only ask "what makes us think we can hear them?"
Part of the point of the Fermi Paradox is that the galaxy should be filled with evidence of interstellar civilizations. A civilization sending out Von Neumann pobes could "exhaustedly explore a galaxy the size of the Milky Way in as little as half a million years." http://en.wikipedia.org/wiki/Fermi_paradox#Alien_constructs
There should be wave after wave of civilizations expanding into the galaxy, each leaving behind self-replicating probes, interstellar networks, and the heat signatures of their civilization. It would be hard not to notice them. Yet there is only the Great Silence.
If they converted a small percentage of their Von Neumann probes to Bracewell probes, which actively seek communication with other civilizations, we should have seen those by now. While radio may be a primitive form of communication for these civilizations, it is by far the easiest and cheapest method. To think that they would neglect radio and only use something like neutrinos or gravity waves is silly.
If we're not the only technological civilization in the galaxy then the only rationale is that they are avoiding us, perhaps for our own good. Still, I think we'd be able to spot a Kardashev Type II civilization from a long ways off, and a Type III might be impossible to miss from anywhere in the galaxy. http://en.wikipedia.org/wiki/Kardashev_scale