My most likely alien scenario is that part of natural evolution is that organic sentients, like humans, evolve technical civilizations that eventually create mechanical sentients. The mechanicals don't require air, water, etc to survive, and may well prefer "living" off-planet. They'd also have a much longer life-span, so to speak, and receive all the energy they require from the light-energy - photo-voltaics or such. Given this, there could well be whole mechanical civilizations existing between the stars or wherever. The Universe is a Big Place. The interests of organics like us would be of no interest to them. And of course mechanicals would rapidly evolve to become an intelligence we couldn't begin to fathom.
We haven't seen aliens because we're just the ants on this one little world.