Because the ghosts are only visible to animals but not human beings bcz something difference should be between humans and animals.
hmmm . interesting question , but may i tell you that science has proved that ghost does not exist in this universe . 'life after death' it is just a imagination of peoples . ghost is just the inner fear of your mind and nothing has no structure which could be given identity .

there is nothing like ghost in this universe 

i hope it helps................. 
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first do u know where is my home
i wouldnt say that ghost exist but souls do has proved it
we can make a complete copy of a person but we can't start it without putting in a special energy
and this energy is soul