Mirror In The Sky
The thing about aliens is that the only thing we know about them is that we don't know anything about them. We don't even know if they exist (Webb thinks that they don't). Recent research explores the possibility of detecting alien civilisations by the air pollution their industries create. It's kind of boggling that we actually have the technology to do this right now, although it only works for planets orbiting uncongenial white dwarf stars, and there might only be a small window of opportunity before the aliens either clean up their act or are strangled by their own effluent. And maybe, unlike us, most civilisations are too smart to produce air pollution in the first place, or perhaps most never go down the industrial road.
When it comes down to it, the question isn't 'why aren't they here?' Instead, it's actually 'are they anything like us?' Could we recognise them, and would they recognise us? If a lion could speak, we would not understand him, but if he sang we might recognise it as song. We hope that aliens might share something with us: music, mathematics, Marxism, motorways. When we search the sky for signs of life, we're really looking for a mirror.