7.1 Extraterrestrial life is rare or non-existent
  7.2 No other intelligent species have arisen
  7.3 Intelligent alien species lack advanced technology
  7.4 Water world hypothesis
  7.5 It is the nature of intelligent life to destroy itself
  7.6 It is the nature of intelligent life to destroy others
  7.7 Periodic extinction by natural events
  7.8 Intelligent civilizations are too far apart in space or time
  7.9 Lack of resources to spread physically throughout the galaxy
  7.10 Lack of desire to live on planets
  7.11 It is cheaper to transfer information for exploration
  7.12 Human beings have not existed long enough
  7.13 We are not listening properly
  7.14 Civilizations broadcast detectable radio signals only for a brief period of time
  7.15 They tend to isolate themselves
  7.16 Colonization is not the norm
  7.17 Outcomes between all and nothing
  7.18 They are too alien
  7.19 Everyone is listening but no one is transmitting
  7.20 Earth is deliberately not contacted
  7.21 Earth is purposely isolated (planetarium hypothesis)
  7.22 It is dangerous to communicate
  7.23 They are here unacknowledged

(via Fermi's Found poetry)