[ WP ] The reason alien never contacted earth is because humans are incredibly big for other species .

After finishing physics civilizations realizes there is only one way to bridge the gap between worlds, and that is the reduction of genetic material to singular celled organisms, no mechanical or multicellular life form could withstand the emptiness of space for long enough to bridge the divide. 
 
 This method however brought it share of difficulties, intergalactic distances meant that not only would innumerable cells need be sent to reach them all, but also that most would never arrive on viable host worlds. 
 
 Earth is one of the rare flukes where the arrival of several alien species actually caused the planet to become viable, sadly the native population will likely cause an extinction level event before ever realizing how close we really are.
