1. severely limited observational ability on the side of humans.
– Until recently humans had trouble to find exo-planets that were not gas giants.
– Humans have only observed a tiny bubble of space around their own solar system.
– Humans are so far unable to determine surface characteristics of exo-planets beyond general geological composition
2. Distance would make alien radio signals indistinguishable from background noise.
– Radio-Signals become weaker with distance (inverse square of distance)
– Radio-signals get polluted with the universe’s background noise.
– Because of distance, the signals might not even have arrived at earth, yet.
3. Alien civilisations’ time frames might not overlap and could be far apart. Basically, a civilisation might arise only long after all others are long extinct, and the last of the signals that might have been strong enough to be recognised as such, have passed earth long before humans existed.
4. Assumptions about alien technology and behaviour.
– aliens might not use radio technology for communication, because of the signal deterioration and the distances involved. Radio signals basically expand in a spherical fashion, which is an incredible waste of energy.
– They might use incredibly focused beams that would never intersect with your planet.
– They might disguise their signals as background noise for security reasons.
– They might use artificial micro wormholes and send their signals through these wormholes for instant interstellar communication, and with certainty, that no one between the origin and destination could intercept the transmission.
5. avoidance of direct contact and intentional concealment of presence, due to probable internal rules of noninterference, to avoid military conflict based on humanity’s savagery coupled with nuclear weapons capability, or to maximise military advantage in a planned surprise attack on Earth.