Charlie Stross' "Accelerando" floats an interesting idea: Post-Tech-Singularity civilizations might naturally become isolationist because consciousness becomes tied to computing hardware that is dependant on abundant energy and low communication latency that you don't have unless you all cluster closely around a star, i.e. a small Dyson sphere. Leaving those ideal circumstances becomes unthinkable for the individual because it would mean lobotomizing yourself.
However, we should be able to see those Dyson spheres, and we don't.
However, we should be able to see those Dyson spheres, and we don't.