You know what? All very nice, but how about this? We are not all that interesting, nor special, and in the last 35,000 years when we could comprehend what we're looking at, no-one's bothered to swing by and ask for a cup of sugar. It may also be possible that we are part of a nature preserve, or that there are more than enough planets with similar conditions to inhabit, to not have to displace or destroy an entire culture. Another possibility is that we're left alone, because other civilizations have been contacted before, and once given technology, have self immolated themselves akin to giving firearms to the natives. That, or we're won the interstellar lottery, and we are indeed the first who will learn a lot of lessons as we swarm across the galaxy once we figure out how to get off this damn rock.
I'm leaning toward the lack of uniqueness about our placement being a significant factor in explaining our isolation. Historically, the more we understood about our outermost surroundings, the less important our position progressively became. Assuming we're nothing special in the grand scheme of things, as has happened before, could that positioning also extrapolate into our biological and technological development?
Perhaps the development of our kind (types of species we're capable of understanding) is nothing special and happens throughout the universe around the same time — plus or minus a few millenniums. If that were the case, in terms of light years, all of our event horizons are still isolated from one another. If we're in the middle of the statistical bell curve, away from being the "luckier" exceptions with well timed positioning near one another, it might explain why none of us know about each others existence.
If true, sometime (maybe someone can come up with a probable calculation when) in the near or distant future, things will start to get interesting.