People see them on the screen so often they think they know them. I guess the term "parasocial relationship" has been common in the last few years to describe this.
I guess for actors and other types of artist specifically, people relate strongly to the work. It can form the basis for life memories. You remember where you were when you heard a song or saw a movie.
Because the service they render brings joy and entertainment to a large multitude of people. It is a higher visibility job than most, and is largely an individual contribution in and of the service they provide
Why do people like fictional narrative so much? I'm not sure why, other than some platitude like "forming narratives is how people understand the world". But I'm not sure why it follows that fictional narratives are so important to us.
I guess for actors and other types of artist specifically, people relate strongly to the work. It can form the basis for life memories. You remember where you were when you heard a song or saw a movie.