And, when you look at how truly short a period intelligent human civilization has been on the planet (500 years, maybe), and how fragile our own civilization really is, you get one of the best reasons for why we haven't yet found proof for extraterrestrial intelligence. The galaxy could be literally teeming with other civilizations, but if they only on average survive for even 3000 years (and even that seems high), then they are only existing for the briefest flashes of astronomical time. The odds of encountering another society diminish dramatically when you understand that the would have to develop almost precisely parallel to even have a *chance* at encountering each other. And even then, they would have to *both* develop FTL travel at basically the exact same moment. If you rely on signals traveling at light speed, you might hear another civilization, but by the time you do, they would almost certainly be long dead and gone.
The only other real possibility is that some civilization makes it off-planet and manages to spread out to other worlds (taking their tech with them each time) fast enough to avoid getting caught in their own destructive wake, so to say. Only then could you possibly see a civilization hundreds of thousands or even millions of years old. And even that is the briefest of blinks at astrological time scales.
I think this is why people like Hawking saw that it was critical for humanity to leave Earth if we have any chance at all of enduring as a species. We are very quickly approaching that critical line for life here on the planet. Sure, life could start over again... but it's starting over from scratch. No forwarded tech. Perhaps it might be worth actually trying to find a way to "pass on" some of our basic technology to a future society somehow. It's a pretty tough thing to do - to make something that has *any* chance of still existing say, a million years from now. About the best we have been able to do so far is around 3000 years. Not only would it have to be durable, but *discoverable* by a pre-industrial society that is still in development (before they start messing things up again).
Good food for thought.