It could be something more mundane. The signs we're looking for are stuff like radio signals, which we'll probably be using for how long? 500 years out of the S

It could be something more mundane. The signs we're looking for are stuff like radio signals, which we'll probably be using for how long? 500 years out of the Sun's 10 billion lifespan. That's a pretty big filter already. Plus, if light speed is really impossible to overcome, then distant stars get less interesting as a civilization progresses. 100 years didn't mean so much for a given population 5000 years ago (not for its individuals, of course), but already are about 3 paradigm shifts these days.