I read somewhere there are at least 6 possibilities:

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

Whatever it is, it’s a fascinating problem and I like Carl Sagan’s approach.