That and the fact that civilizations probably only broadcast EM radiation out into the void for a short period of time before they start getting into energy efficiency and switch over to more efficient communication technologies.
Our television and radio broadcasts aren't detectable by an Arecibo-class telescope out much past Jupiter let alone outside the solar system. Even our high powered radar systems wouldn't be detectable out even half a light year from the solar system.
The only civilizations that can be detected with a SETI-like program would be ones intentionally transmitting directional signals. Even then out past a thousand light years even a multi-terawatt (EIRP) signal would be difficult to detect.
Like inverse square law is as unforgiving as the rocket equation. Anyone hand-waving either of those principals is not trying to have a meaningful discussion about interstellar communication or travel, they're just writing about science fiction.
Not necessarily. Gravitational lensing may enable directed communication at much lower power levels : https://storkpaulo.wordpress.com/2010/04/26/gravitational-le...
Say we had an SGL telescope positioned such to image an Earth-sized planet we know is in orbit of Alpha Centauri A. For ease of math we'll say this Earth sized planet is at a distance such the solar constant is the same as Earth. So that's a total power received from the Sun of 1.74x10^17 watts and with the Earth's mean albedo of 0.3 about 5.22x10^16 watts being reflected off into space.
We can use our SGL to image the planet around Alpha Centauri because it's reflecting 52.2 petawatts of sunlight out into space. So it takes the power of a star reflecting off a disc with a cross sectional area of 1.26x10^8 square kilometers for a proposed SGL system to detect and image a planet.
The amount of power an omnidirectional antenna can possibly emit is somewhat less than 52 petawatts. Broadcast antennas output at most a few megawatts EIRP because there's no utility in blasting out hundreds of megawatts for terrestrial transmission. Such broadcasts just aren't going to be detectable even with gravitational lensing from our Centauran neighbors.
Nor are gravitational lenses terribly useful for beacons since you need the right geometry between the sender, star, and receiver to use the lens. You could use lensing to increase the EIRP of your transmission but only at a specific target. Nobody outside the focal plane of the lensing system is going to benefit from the lens.
SGLs are a neat idea and an interesting topic but I don't think they solve many SETI problems.
You look at human civilization and that's exactly what's happened. We started off by broadcasting everything as loud as we could (which isn't very loud) and we've slowly transitioned to signals that are both quieter and don't really survive escaping the atmosphere.
I can't imagine an advanced species wouldn't follow a similar communications path. The only way we detect them is if they specifically target our spot in the sky with focused EM for a very long time, generations! After all, they have no clue when we would be in our evolutionary path.
And for that to happen, the species would have to first find us. Not just find us, find us while we are still listening.
And this is all with the assumption that FTL communication/observation is even a possibility (It likely isn't).
IMO, the simplest answer is that FTL communication and travel is impossible. Advanced civilizations across the galaxy have all come to the same conclusion.
We might be able to explore our own neighboring stars throughout generations but it's unlikely we'll be able to ever send a message to another civilization that will get there while they are listening (without spending huge amounts of power).
After a long while, there might be this great replacement, totally peaceful, that would lead these conscious organisms with a vastly superior intellectual efficiency to probably become very small and self centered and not behave like the territorial monkeys we are. What do you think a massively singular electronic intelligence will think when it notices a planet like ours ? "nuke them all and rape their women" ? This is monkey behavior that we still haven't fully shaken up.