I think they sort of cover this but here is my simple explanation. Think about how many slugs there are on Earth. How many of them have never seen a human? If w

I think they sort of cover this but here is my simple explanation. Think about how many slugs there are on Earth. How many of them have never seen a human? If we are slugs in comparison to advanced beings that have a billion years of evolution on us then it is no surprise we have not seen them.