Fermi's Paradox. There are so many stars (more than there are grains of sand on earth), that the probablility that one of them has life, and even intelligent life, is >99% . So why haven't we observed it yet? Cue a lot of brilliant people trying to answer that question.
The Dark Forest - no one wants to alert their presence or attract predators. Though knowing our Earth I think we're stupid enough to do that. Cue the space lasers.
It could've also been knowledge interstellar species gained through experience too: if in their first encounters they were either wiped out, or nearly wiped out, then they're not going to reach out again.
What you observe of the universe died a really long time ago, it's improbable that other intelligent life in the universe can observe us and the same with us.
We could be multiple galaxies away from each other and never ever know of each other.
Space is big, light is slow, and the inverse square law is a thing. You think we've been pumping out radio broadcasts for hundreds of years and nobody has contacted us yet, but we're only detectable to life within 200 lightyears if they're specifically looking for the signals we pump out, and they're looking exactly at us. We'll only see a response if they decide to, and we can detect it, and we're looking at them when their response reaches us, and we recognize that it's a response and not a peryton.
Not really. The paradox is based on the idea that there are so many stars that even if an infinitesimal portion have intelligent life who have discovered radio, the universe would be much noisier than it is.