They think the known universe is just shy of 15 billion years old. That’s a long time. Compare that to how long humans have been around — approximately 2 million years at the most. That’s not even 1 second on the time dimension of the universe.
But if you were to look at the night sky without any light pollution, you would see just gads — that is countless — stars near us. And then again, we now know there are countless whole galaxies out there. Given the time dimension of the known universe, it raises the very likely scenario that there are civilizations on other planets, somewhere, that have been around not for merely millions of years, but perhaps even over a billion years.
Just think how far a civilization might actually advance in knowledge/technology in over a billion years…when you look at how far we have come in merely a couple million years? A civilization that has advanced in over a billion years would probably see our civilization as comparable to the way we view ants…not even ants.