Self assembling replicators from some civilization should have colonized and cataloged every star system over billion year time scales.
It's inconceivable that no intelligent space faring civilization would have thought to do this.
There are too many stars and too many close enough galaxies that could contain life. If they've been spacefaring for even ten thousand years longer than us then they're almost certainly able to monitor us without our ability to detect it.
But if we're doomed to a great filter in the near future, like nuclear war, then it still doesn't mean we're alone. It just means that out there in the sky there are others kinda like us. Searching for meaning before hitting a technological explosion of one kind or another that wipes us out. But just because we don't have contact doesn't mean we're alone, even if we're only present in each others respective imaginations.
It's entirely conceivable that every space faring civilization which has tried has failed simply because the odds are so stacked against success, even once, at every step along that process. Just because the math is simple, doesn't mean the application is simple.
Hell, we don't even know what the odds are of becoming a space faring civilization to being with, and it doesn't seem axiomatic that every such civilization would even want to seed the galaxy with self-assembling replicators to begin with.
What if the ROI of space colonization even at multi-decade or multi-century timescales is negative? Then why would anyone do it?
What if they just feel like exploring, and gaining knowledge, and seeing things no one else has ever seen?