So far we haven't found evidence of other civilizations out there. Of course, there are lots of good reasons for that, not excluding what you've pointed out. There's also an important time factor: Any species only lasts so long. A million years would be an exceptionally good run. So it is more than possible that intelligent species have flourished and died off, and that in our time we simply won't co-exist with any other intelligent species.
Another factor that seems to me to be generally overlooked is the vast distances between stars, which may well make genuine interstellar travel just not possible.
Here's the best illustration I know of about interestellar distances: Our galaxy, Milky Way, has about 300 billion stars. The nearest galaxy, Andromeda, is about three times our size, with one trillion stars. As you no doubt know, Milky Way and Andromeda are on a collision course, and will intersect in about 4 billion years or so. I recently asked my favorite astrophysicist (who happens to be my son) about how man stars would crash into each other at that point. He said, no more than 10. THAT, more than any other example I know tells you just how vast interstellar distances are.
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