Why haven't we visited by aliens yet?
- We're the only planet with some sort of civilization
- Inhabited planets are too far apart for space travel, due to speed of light restrictions or something else
- Civilizations advanced enough to build ultrafast space ships also don't want to visit other planets (for moral reasons, or maybe they think it's too boring, or they developed an advanced cyberspace to simulate anything, including space travel)
- Civilizations across the universe evolved at about the same speed following the big bang, so other planets are still building better space ships like we do
- All civilizations that evolved to the point where they might soon build ultrafast space crafts blew themselves up with nuclear weapons etc.
- We've been visited by aliens but they (or we) keep it secret (or our memories of it will be erased after visits), or we are in fact alien descendants ourselves