I read somewhere there are at least 6 possibilities:
- Dark forest theory - read at least book 1 and 2 of three body problem series or google for spoilers
- A filter exists, or more plainly something that always causes a civilization to eventually fail.
- Similar to Star Trek, other civilizations are waiting for us to pass a threshold before contact is made.
- Carl Sagan: the universe is so large and intelligent life is so sparse, we just haven’t come across them yet.
- Life on our planet is unique to the entire universe
- Could also be a mixture of some of these.
Whatever it is, it’s a fascinating problem and I like Carl Sagan’s approach.