So previously I dealt with the issue of why aliens haven't come visiting but reading this article at Cracked written by ISS astronaut Chris Hadfield(Note some NSFW language in the comments section) and I realized that there was one assumption that I hadn't addressed; that our eponymous aliens could actually travel into space to begin with.
It is in fact pretty astonishing that humans can survive in space, and I'm not talking about the machinery that provides air and water and heat to keep the astronauts alive. We are a species that evolved on a planet under 1g of gravity. When we travel into space that gravity goes away and somehow the human body puts up with the this. Your bladder may not work the way nature intended, your reflexes and instincts are all completely wrong and have to be relearned and yet somehow people manage.
Now barring miracles of technology an alien race is probably going to be following the same path as earthly space flight. That means chemical rockets launching cramped vehicles whose crews will have to endure zero gee on their flights. What if they simply can't endure? What if the myriad of symptoms that sometimes afflict human space travellers were endemic and permanent for our alien friends?
It maybe that with sufficiently advanced technology the aliens could overcome such difficulties but what are the odds they would ever make the necessary breakthroughs if they never take those first steps?
Perhaps the galaxy will belong to mankind not because of our intelligence but because of our cast iron stomachs...