Why does Science refuse the existence of some creatures?

I was watching lost tapes. They were talking about the Chupracabra. At the end. This little girl that survived the "attack". The announcer said she had nightmares about the creature that science refused to accept. Why can't science accept animals such as these. They're just like other animals except a bit deadlier. There are a bunch of other animals that are deadly. People have sighted the "fiction" creatures. Why can't they accept it?