They don't, really - determining what properties "a normal" universe "should have" by examining the only universe we know about doesn't tell us much about any universes we don't know about. It could be that universes are things that happen all the time, and the ones that aren't suitable for our-style life? All we can tell is that *we don't live in one of them*. Put another way: Out of our sample of one, all universes eventually produce human life, without any evidence of outside intervention.