Guys, 2 issues here, the Drake Equation (estimate of number of earthlike planets in the galaxy) and the Fermi "Where are they" Paradox. In its most succint form, the Where are they? paradox may be rendered as [courtesy Dr. Jill Tarter]:
1. IF extraterrestrial civilizations have existed elsewhere and "elsewhen" in our galaxy,
2. AND IF interstellar travel/colonization/migration is
inevitable for at least one of them,
3. THEN simple calculations indicate that an expanding wave of colonization will fill the galaxy on a time scale short compared to the lifetime of the galaxy,
4. BUT we do not "see" them here,
5. THEREFORE (1) is wrong; there has never been another technological civilization anywhere or "anywhen" on our galaxy except the Earth!
As regards point 2, I would like to turn it on its head. If we posit that technological civilizations do arise fairly frequently, AND if the galaxy is not filled by [at least] one of them, then for that to be true we require that ALL individuals in ALL those societies eschew space travel for the ENTIRE existence of those civilizations. This is highly unlikely, to say the least.
All this assumes that interstellar space travel is possible: my feelings are that for a civilization that has harnessed the entire resources of its solar system (including its Oort cloud), it is.
First of all, there's every plausibility that life is common but that technological civilizations
do not arise fairly frequently. Out of millions and millions of species on Earth, only
one has developed a technological civilization. Moreover, that particular evolutionary tree took some 600 million years in order to accomplish the task..
Secondly, you again assume not only that other species would have personality characteristics conducive to interstellar expansion, but also that interstellar travel, migration, colonization, and communication are all relatively plausible, effortless, and attractive.
Consider if you will, an Earth where near-luminal travel were available - tomorrow. How many people would want to take such a trip? Where would they go? Let's say they hop on over to the next earthlike planet. They would be essentially cut off from Earth. One communication signal would take at least a decade to travel back and forth.
What would they do once they're on that planet? What if the planet turns out unsuitable? What if they have a mutiny along the way? A civil war? They have to establish the colony. For what? Are they dissidents, or whatever? Now, how long will it take for them to set up a another expansive, resource gathering civilization? In order to send out more colonies. What are their motivations? Or, do they wait for this new world to fill up first. What if their civilization collapses?
The questions proliferate ad infinitum. The fact of the matter is that: we do not see indications of other civilizations, which suggests that other civilizations do not expand to fill the galaxy, and we don't know precisely why that is...