“why, in a universe vast and ancient enough to host billions of potentially habitable planets, we have not detected any signs of alien civilizations.”
Our solar system is essentially in the boondocks galactically speaking.
It’s the equivalent of an Iowa farmer going out on his front porch and going “Well, shit, I can see for MILES and I don’t see any evidence of any ‘New York City!’”
There are LOTS of reasons we might not be able to detect alien civilizations, #1 being we’ve spent entirely too much time looking at radio waves.
As of last year, we’re on the edge of having a communications system that doesn’t use radio waves: