(i) Maybe it's the problem of entropy - everything heading towards equilibrium rather than 'order'... i.e. it's hard to get life to evolve (although if it has happened here, perhaps statistically speaking it is likely to happen elsewhere).
(ii) If they do exist and attempt to communicate would we understand?
(iii) maybe it's anthropocentric to think other life-forms would be remotely interested in communicating with an alien species even if they knew we exist.
(iv) maybe our instruments of detection are not sufficient to detect alien life forms.
(v) Worst scenario, we might appear like insects to more advanced alien life-forms - swarming the planet and if they came and saw us destroying the planet they might want to do a 'rentokil' job on us.