1) Earth is located in a region of the Galaxy (Orion's Arm) where alien activity is low.
2) Aliens, most likely, are different, some do not want to touch us, and some are fascists. We're probably just lucky. Maybe even some kind of "kind" civilization defended our "reserve" from the aggressive and warlike hordes, and we will never even know.
3) Also, maybe warp drive is not possible, and therefore aliens colonize territories extremely slowly, and given the average distance between planets with intelligent life, contact between civilizations is extremely unlikely.