I would say the best word for that spot would be obsessive, as interested sounds more passive than I would like, fascinated sounds like the positive version of obsessive, and passionate also implies something positive, as you are passionate about a hobby.
The definition of obsessive from Google is "a person who is affected by an obsession", and the definition of obsession from Google is "an idea or thought that continually preoccupies or intrudes on a person's mind".
Obsession of something is usually looked down upon in society, as an obsession with a particular person is seen as creepy, hence why I decided to use this word. Going back to my hobby example, people normally say they are passionate about something, like guitar, but never obsessive. If I was to say "I am obsessive about playing guitar", I would likely get a lot of concerned stares.
The sentence would be essentially saying "The media cannot report on anything except for people who can only talk about the personal lives of celebrities and politicians".
Your final sentence would therefore be:
The mainstream media are obsessed with reporting the gossip about celebrities and politicians.