And Earth is a place where pretty much everything stemmed from the same evolutionary trunk. We are far, far more closely related to octopi or centipedes than we would be to anything that evolved outside our biosphere. In fact it's pretty likely that we may not even recognize alien life as being alive, at least in the sense we are used to. A silicon based or argon based life form would exist and operate in a completely alien manner (heh) to the carbon base that we're used to seeing.

Moral of the story being, the chance of humanoid aliens existing is a miniscule fraction of the chance of alien life existing at all. Much more likely that we'd scan right past an alien lifeform and quite possibly not even notice it was there, especially if we're busy scanning for carbon-based Earth style life forms. Gotta keep your eyes and your options open, because the end result of a completely different evolutionary timeline will inevitably produce a completely different style of creature.