Intelligent civilizations’ lifespans are short and don’t overlap
Compared to the relative age of the universe, we’re all here for a mere moment. According to this Reddit user, “The time span between when a civilization becomes capable of transmitting radio frequencies and when that civilization annihilates itself is so short that we seldom overlap.”
I mean, it makes sense. We just discovered radio waves in the 1880s, and have yet to crack interstellar travel, but we might just kill ourselves or destroy the planet, before we can really get out there.
Which leave the next intelligent civilization to wonder where everyone is.