It costs them nothing to wipe us out and spare the universe the trouble, so what must they believe about life, the universe, and everything to not do so. Economics may be universal, etc.
Perhaps the world is highly valued in terms of its bio-diversity and life-supporting resources. There's a possibility that native civilization like us are being monitored and on some level "protected" from conquests if they show they're responsible and take care of their own world.
But if the native civilization continue to destroy their own world, then all bets are off. More responsible and structured civilizations could be granted the right to intervene and potentially take over.
In other words, the more self-sufficient, ecologically responsible and wise we are, the more a civilization like us would be able to avoid this intervention.