Perhaps most advanced intelligence comes in the form of distributed hive minds, and something like a "civilization" is regarded by most sentients as something akin to a pack of wolves or an uncontrolled mob? Maybe we're the ultimate ethical dilemma: All of the bad qualities of a cancer, but you can't ethically kill us because every one of our units is an independent sentience.
Our neighbors are quietly watching us, hoping we'll choke in our own industrial effluents and kill ourselves off in the resulting ecological chaos, so no one has to taint themselves with the unpleasant task of sterilizing our system.