There’s also a significant possibility that we don’t really have the proper instruments (yet?) or know exactly how to look for the other civilizations. We could, of course, recognize a civilization that has the stage of development compared to our own, but I have doubts this stage is very stable and lasts long enough for such a civilization to be easily noticed considering the vastness of the Universe.

The lesser-developed civilizations (consider humanity about 200 years ago — merely 200 years!) should be effectively invisible to someone observing even from the closest star. As for the more developed ones, we can’t really be sure what they are supposed to look like, can we?

Also, a couple of papers I stumbled upon last year seem to suggest there can be from seven up to fifty-three candidates for Dyson spheres among the visible stars around us, so maybe the other civilizations are right around us while we fail to notice them and proudly think of ourselves as the most advanced species out there.