Why don't people believe in mythological creatures?

Like mermaids, dragons, fairy's and Nessie how come if you say a mythological creature people are just like 'grow up you childish moron'? They probably do exist but have very good hiding places. or humans hunted them to extinction or something... And also why is it that like in all those films when someone finds a mermaid, why do people always want to dissect them and cut them up? i would just be happy to see it with my own eyes, and, hell, to talk to one would be amazing. why would people want to cut up beautiful, mysterious creatures just so they can see what's inside? people always kill things they don't understand. like the na'avi in AVATAR.