Why are so many people obsessed with following celebrities lives and gossip about it?
Like, why do you care so much if she's sleeping with a new guy after just divorcing or her coming out or that rich family buying a new mansion? How does that affect your life?
In short: their brain chemistry tricks them into thinking that they are following a friend and have the emotional reactions and interests as we'd hope our real friends do.
It's how the Disney channel operates. They hook kids young with teen oriented sitcoms and then the viewers get to "grow up" with their favorite actors over the course of years.