If an alien civilization is on a planet that's, say, 1000 lightyears from Earth, and they even have telescopes good enough to see Earth, they'll see Earth as it was 1000 years ago, stet? There won't be any technological civilization for them to see signs of. Same goes for if they have radiotelescopes; no electromagnetic emissions for them to detect. So realizing this, why would they even attempt to contact us?
The reverse of the above fully applies to us trying to detect *them*.
