That's the biggest factor. They rely on big name stars, instead of quality storytelling.
With the price of a movie ticket being so high, people aren't going to see a movie, just because a certain actor is in it.
Note that the most popular TV series have no top tier actors in them. "Top tier" referring to pay-line, not talent. I often think that having a known star places another hurdle on the storytelling, as the audience must set aside the fact that they already know the person. Instead of easily accepting that they are following the life of say Colonel Claus von Stauffenberg, the audience is constantly reminded that its Colonel Claus von Stauffenberg, as played by Tom Cruise. When the audience doesn't know the actor, they quickly accept the character being genuine without distraction.
Absolutely true. Kevin Spacey has said that he doesn't want people to know anything about his private life because he wants to be seen on screen as the character and not the actor.