Obviously we can't tune in live to the entire galaxy in every direction at one time. But if you ever stood out on a brilliant starlit night in a very dark area, most people haven't- it's obvious why you don't have to.
We only have to search within the galactic plane, which by definition is a very limited portion of the sky, and within that, the densest regions obviously offer the best odds. And ET would also look for us in that narrow plane, in a nice quiet suburb, nice main sequence single yellow star system.. then see nice stable orbits, protective gas giants, an inner rocky planet in the habitable zone.. one thing would lead to another
Our planet has presented an ideal piece of vacant real estate, for hundreds of millions of years. If civilizations were at all common, they would have spotted this for sale sign a 'galactic' mile away- not only would we have heard or seen them, they would have been here personally a long time ago (ancient alien theories not withstanding).
The belief that they are out there but hidden, relies on the further belief that not one single civilization ever developed the capacity to colonize, even though it could have been done with tech. little better than our own, many times over by now-
or that some galactic treaty prevents disturbing uncontacted tribes - not impossible but it's walking a pretty thin line.
But the observation merely supports the math, the odds of another planet like Earth existing in the universe, far less our own galaxy, are not too great.
