My own favorite hypothesis goes like this: Our universe is most likely to be the simplest one that contains me (us, observers, conscious beings, whatever your favorite rendition of the anthropic principle). It is not likely to be much larger than necessary for creating me. The reason it is as large as it is, then, is that that's what it takes. The answer, then, is that something like me exists only once. More would be a waste of universal size and/or complexity, and Occam forbids it.
Is this as crazy as it sounds?
Yeah, pretty much. It would be my default assumption, but only if I was completely ignorant about anything beyond the atmosphere. And if we're going to put ourselves in that position, it's not entirely unreasonable to conclude that Marduk grew the world from a weed.
If you are referring to complexity, then I think it's almost common sense.
On the abundance of extraterrestrial life after the Kepler mission Amri Wandel
Some recent calculation of the Drake Equation with estimates of the likelihood and logevitiy of civilizations
Related: The Great Filter and Planets in the habitable zone, the Drake Equation, and the Great Filter