JK41R4: I don't think there's a problem with casting ScarJo, but I agree that this can be a vicious cycle. And sadly, it's either Hollywood that doesn't want to break it, or the movie-going public who may only want to watch movies with already famous actors.
Considering Hollywood bemoans that franchises are the new box-office draws and they can't sell a movie on an actor's star-power alone anymore, I think I place the blame more on them than the public actually, though I'm certain at least in a small part the blame lies with them as well.
It's what Falci said above
Falci: Hollywood, like any other industry, was built on market segmentation and is allergic to changes
As it is now, I think the problem lies in considering white the "default race" when casting roles.
When a character is written from the ground up to be of a specific ethnicity they will cast a non-white actor, but these roles are hard to come by and are often mired in stereotypes.
When they write someone to whom their race just isn't a part of their character at all, which I believe is most of them, instead of thinking "We can cast the best actor for this role, whatever race they may end up being", they think "They didn't specify the character's race, that must mean they're white, we'll cast a white actor then".
If they allowed themselves to do genderblind casting on characters where the race doesn't matter at least some of those parts would go to non-white actors, these people would be able to carve a niche for themselves, maybe starting small on TV and eventually when there is a part meant to be played by a certain ethnicity we would have well-known actors that fit the bill.
Johnny Depp may be doing caricatures of native-americans on the big screen on the back of his massive fame
now, but he started on the 21 Jump Street series on TV back in 87.