On top of all that, we're forgetting that aliens might not just be very distant from us in space, but also in time. Implied in the Drake equation is also the percent of technologically advanced intelligent life-bearing planets in the universe, but technologically advanced intelligent life-bearing planets that happen to have hit some technological threshold within the right time frame to contact us. Once you start thinking of it that way, the probabilities get freakishly small. A couple small evolutionary events and a parallel Earth could have achieved sentient space-faring intelligence of some sort a billion years ago. If they came to check us out we wouldn't be here. Likewise, a couple mass extinction events on another parallel Earth could keep space-faring intelligence on the back burner for another 2 billion years.