It seems like you're assuming the difficult part about creating self-assembling replicators capable of sustained, error-free exponential growth across interstellar space and billions of years is merely coming up with the idea.
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.
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.