First of all, we are talking about predicting the actions of superintelligent aliens. We aren't talking about slime molds or a bacterial infection, but beings with more intelligence than us, like orders of magnitude more complicated reasoning processes. They are effectively unpredictable, because if they weren't, wouldn't we be just as smart as they are? That level of intelligence comes with new levels of reflectiveness that are frankly not accessible to our weak brains. Like, maybe after reaching the next level of enlightenment, they decide that expansion throughout the universe like a worm is not actually what they want or that is what is good for the universe? Maybe they decide to rather study the universe and not interfere, the way biologists study, say, apes, without wanting to disrupt their natural behavior with their presence? Or maybe superintelligent beings find a way out of our universe, and pass into another dimension (cyberspace? black holes? disappearing through quantum foam? heaven?). Or maybe something else insane happens, like being superintelligent inevitably makes you completely terrified of everything, because you realize how powerful other aliens are and your calculations unequivocally show it's safer to stay at home and fart around doing the super-intelligent analog of baking chocolate-chip cookies? Not being superintelligent, we are like ants trying to contemplate human civilization. We have no context or experience, and we simply don't have the mental capacity to even imagine what we don't know.
Second, these probabilistic models are so fundamentally stupid. It's impossible for us to reliably estimate the probability of any event that's happened only once (without reasoning about the process that gave rise to that event), as we cannot estimate posterior probabilities without lots of samples. And humans on Earth have a massive chain of single-occurrence events that all seem to be absurdly improbable. Besides that, and perhaps even because of that, it's not even a foregone conclusion that our universe even selects from the middle of the distribution (i.e. is not biased). We could perfectly estimate the shape of a probability curve and yet live in a universe that stubbornly selects from the end of the distribution, all the time. All of our math could be totally right, but crazy shit just keeps happening.
Third, the anthropic principle is a total bitch. It means that we will never be able to estimate probabilities of events that were required to create us, because no matter how improbable they were, if they didn't happen, we're not even here to argue about it. If there are infinite universes (a multiverse, see Tegmark), then forget this whole mathematical sophistry, because then there is no need to live in some "average" universe; we can easily live in some stupidly improbably snowflake universe that has the 90 trillion improbable things that make apes possible; infinity doesn't care.