The main problem with the proposed solutions is that they don't explain the COMPLETE lack of aliens visiting us. See Drake's Equation.
- Konrad Lorenz's point that the only reason organisms don't kill other members of their own species (fast enough to go extinct) is the co-evolution of natural weapons (e.g. teeth, poison) and behaviour (aversion to using the weapons, like we don't enjoy biting people's throats out). Only one Earth species has ever developed new weapons much faster than it can evolve new behaviour, and that's us. And we should expect the same for other newly intelligent species.
- New idea: maybe an extreeeeeemly predatory creature will escape this by evolving (before they invent technology) a very general aversion to killing conspecifics (members of the same species) in any way.