I'm okay with the Field of Dreams interpretation of astrobiology: "If you build it, they will come", so to speak. All you need is for life to take hold once and it'll take over a planet in just a few billion years. Of course, my own hunch is that suitable chunks of rock (planets, moons, etc) are somewhat more rare than the default value[1].

[1] I plugged in .1, among other variable changes. On the other hand, I got the number of communicating civilizations = .02, so either I'm too pessimistic or we're just really, really lucky.