[Ken H]

If there was even ONE other planet with intelligent life, one would have expected that civilization to have eventually expanded outwards to other systems...

I think civilizations eventually 'virtualize'. It's just a matter of time before virtual reality becomes so realistic that people will want to stay there all the time. What does outer space have that can beat creating and living in your own world?

[Boogieman]

“What does outer space have that can beat creating and living in your own world?”

Sexy alien women maybe? That seems to have been Kirk’s primary motivation.