Another factor not in the Drake equation:
You need tides for life. That means you need a massive moon, relative to the host planet. Earth, so far, has the only such massive moon. There are moons on Jupiter larger than ours, but Jupiter is so much massive, that they are puny relative to Jupiter, and as such were simple for Jupiter to catch. Earth catching the moon by gravity might be something like sinking a hole in one from 10,000 yards without hitting the grass. The present theory for the existence of such a massive moon around such a modestly-sized planet is that it’s the result of a direct (yet glancing) collision.
But a collision with what?
“But a collision with what?”
Another restrictive condition... whatever hits the planet has to be big enough to knock off a significant chunk of mass but not large enough to obliterate the whole planet.