In light of these new discoveries, the Drake Equation suggests the galaxy ought to be TEEMING with aliens. So why haven't we met them?
Well, here's one dark-tinted answer: Does a galaxy filled with habitable planets mean humanity is doomed?
Other popular answers include:
Advanced civilizations don't use messy radio. Even our sphere of radio "pollution" is fading as we move away from that mode toward tight-beamed information and fiber.
Technological civilizations don't last long before they self-destruct. We might be proud of our nation, but the oldest continuous civilizations on this planet have durations in the thousands of years - that's just an eyeblink in the timescale of the galaxy... and we have only recently (in living human memory) invented ways to self-annihilate. Millions of equally advanced civilizations could have appeared and vanished before the Earth was even capable of supporting life.
On a related note: If a civilization is capable of creating the Matrix, they will. Animals seek comfort, and intelligent organic life is still a comfort-seeking animal. How many of you feel you could resist the siren song of everlasting immersion in a simulated (but absolutely realistic) world that satisfies your every need and desire? Heck, we could be living in the Matrix right now and not even be aware of it. If advanced civilizations go this deeply inward, they won't travel or communicate outward.
Advanced technological societies will always create AI, which will supersede them. This is the notion of the Technological Singularity. Relates to the prior notion if AI is benevolent, or to The Terminator or Berserker series if not. Good luck fighting something a million times smarter and faster than you, should it decide to eliminate you. Or save you to extinction, a la The Humanoids.
Planet-sweepers abound. Asteroids polish advanced life off the surface of the Earth every so often, supervolcanoes erupt even more frequently (and volcanic activity is important to creating life), even timid stars like ours go through periods of massive activity, supernovae eradicate life in their stellar neighborhoods, viruses and bacteria evolve much faster than complex life....
A Galactic Prime Directive that makes advanced civilizations invisible to the rest of us. This requires a massive bureaucracy and police force, and a population easily controlled, but it's possible. (Hint: This is the reasoning I use in The Adventures of Jack and Stella.)
They're talking, but we just can't decipher it. SETI mostly looks in the radio bandwidths, but why would super-advanced civilizations use such backwards tech?
No one has figured out faster-than-light travel. If they can't move around and colonize, we wouldn't have met them yet, and they'd be less likely to survive a planetary catastrophe if they're confined to one or a few worlds.
Or maybe everyone is just afraid of everyone else, so they're out there, everywhere, but quiet, afraid to announce themselves. If they are like us, first-contact situations don't end well, and there's no rational reason to believe everyone you'll encounter is less-advanced than you.
Do you have a favorite reason that explains why 1) the galaxy isn't teeming with life, and 2) if it is, why we haven't yet detected it?