Why doesn't replication protect you against a bad write or an accidental mass delete?

Replication propagates every write — good or bad — to every replica, typically within milliseconds, with no delay window in which a mistake exists on the primary but hasn't reached the replicas yet. Only an independent, point-in-time backup lets you recover to a moment before the mistake happened; replication faithfully reproduces the mistake everywhere just as fast as it would a correct write.