@acfusco @manton There is plenty of space and time for other civilizatios to rise and fall, and never touch us. Even if a civilization spans a big part of the galaxy and lasts a million years there is still plenty of time and space for that civilization to disappear and never leave a trace that we can find.
Another possibility is that the chances of intelligent life evolving is so incredibly rare that it's only happened a few times in the universe, even though there are billions of Earthlike planets. Consider the number of snowflakes in the world, and yet every one is unique.