Time. No one knows how long a technological civilization lasts. The 300 - 400,000 civilizations may have happened 100 million years ago or a 100 million years in the future.
Technology: An earth type planet could get stuck at the dinosaur stage forever. Or even if intelligent beings evolve they could get stuck at the Roman Empire technology level for ever. No guarantee that they will ever discover radio or space travel.
Time.
As vast as is the universe, Time is even more vast.
We have had broadcasting technology for less than 200 years. Let’s call it the “radio age”
What are the odds another civilization’s “radio age” results in a broadcast that travels over millions of light years to exactly correspond with our ability to detect it.
You are correct, “civilization” was an overstatement, because of the factors you mention, life, itself however, is probably ubiquitous throughout parts of the galaxy and universe. Indeed, we may yet discover it throughout the solar system.
To be certain, I have no expectation, that humanity will ever discover or have meaningful contact with other intelligent life, but our level of technology and understanding forces a constant reappraisal of our pre-conceptions.