Most humans don't believe in something they can't/don't see. But you must think, why wouldn't those things be afraid of humans? Most humans tend to kill the unknown. This is shown by the short story, The Large Ant by Howard Fast. This story is a story that tells about a guy that discovers a "large ant" looking up at him on the end of the bed. Before the guy thought, he took a golf club and killed the "large ant" which was actually an alien that resembles an ant. So the guy took the dead alien to a museum that has had 8 aliens previously brought to the museum. They were all by "accident" but it is just human instinct to kill something that we fear or is unknown to us.
Remember the Salem Witch trials in Massachusetts? Humans killed many people that were accused of being a witch. They killed them without hearing their story. Humans tend kill things without a real true meaning besides fear.
Maybe mermaids are real but they just fear us.
Many times Discovery Channel has found the skeleton on 'mermaids'. Would you believe it?
Discovery Channel
CNN believes that Discovery Channel is faking it. Do you believe that they are?
CNN