Why do some people believe in mermaids?

It depends on who they are. A young girl might believe that they are real, but after a certain age she should know that they aren't real and only make believe. Some people want to be removed from reality so they believe in things that aren't real. Mermaids, faerie's, vampires and aliens are just some of the things that people displace reality with. These people need help. So, there are many reasons why they would think mermaids are real.