Saying Clooney isn't a star is pretty ridiculous, since he, and he alone, opened The American.
When I bring up someone like Johansson or Crudup, I think of people that are famous, and have been enjoyed/appreciated in certain films, but who simply cannot carry a film. I think Johansson is smart, gorgeous, and talented, but I have never gone to see a movie because she was in it, and her presence alone has in no way caused me to be me interested in a film that I would normally never be interested in.
That said, there are so many contributing factors to the failures / underwhelming performance of the films of supposedly big actors, that it's kind of dumb to say that it's all about people not caring about "big stars", anymore, but I do have one idea that sort of lines up with this.
The double-edged sword of too much access. You can totally avoid hearing about this and that actor's sex tape / drug problem / political ideology / religious affiliation / break-up / hook-up / fashion faux pas / favorite cereal, and so on and so forth, but it permeates popular culture, and it is suffocating. Consciously and unconsciously, I think people are responding to that, they always have, it's just way more pronounced now.
That very thing has completely fucked up Tom Cruise's career, and at the very time when he needs it the least, when he's getting fucking old.