One thing I've though about is that it isn't just intelligence that matters. It's also available resources. In order for a galactic civilization to spawn, the intelligence must evolve on a planet or group of local bodies with natural resources sufficient to allow it to spread. Spacecraft have to be built out of something. Fuel has to come from somewhere.
I wonder if there have been countless other lifeforms and civilizations that evolved and then subsequently died out because they used up all the natural resources available to them.
