Why are myths, folklore, and mythical creatures very famous in movies? Why do we like to watch such movies?

Good question.
There are two very different answers to this question. The first is that the deep structure of myths is built into the way we tell stories. Some scholars have argued it is built into our minds. We see things in events in terms of the hero's journey. We look for these narrative structures in our experiences, and use them to make sense of what we go through, or what we see others going through. In the movies, some of these structures and mythic elements occur naturally or without conscious intent. Other times, as in Star Wars, movie creators consciously use mythic structures to structure their films in ways that generate stronger emotional responses.
 
The other reason myths and mythic creatures show up in films is more basic. Movie makers are always looking for new thrills, new threats, and new gimmicks. This includes looking for new monsters. Writers therefore pull creatures from myth systems around the world in search of novelty. Ancient monsters become the next new thing.
 
http://users.clas.ufl.edu/agordon/starwars.htm

https://ccas.creighton.edu/