Either intelligent life is rare in the universe or there's something that happens to it that prevents it from what we envision as the future of our own species, i.e., colonizing and spreading among planets. Maybe 99.99% of species ruin their own climate before getting off-world. Maybe they all decide it's happier living as hunter-gatherers. Unless we're alone, there is some fate that's common to all intelligences and it isn't a Star Trek future. The universe is simply too old. If anything else like us existed and it was possible to exist like us for a long period they'd have filled every planet before we ever evolved.
Personally I think just the fact the universe hasn't been eaten by Von Neumann machines means we're alone.
Personally I think just the fact the universe hasn't been eaten by Von Neumann machines means we're alone.