Instead we've got all these people losing their minds about a Caucasian looking woman cast as the Major in Ghost in the Shell when the director and the Japanese audience are fine with it. Despite the point that the "ghost" (one's consciousness/soul?) and the shell (what body you wear) are a big part of the entire point of the thing. Are bodies (shells) as important in that society as picking your outfit is today (i.e. they're what people see, they're what people judge you on, but it says more about your taste than who you are as a person)?
If you want to pick a different movie to lambast, try Death Note. It *could* easily be adapted, as the story affects the entire globe, but as written it's clearly set in Japan. L is a character of mystery, who could be played by practically anyone, but Light and family and the task force are Japanese. So if you want to complain about "staying true to the source material", then complain somewhere where you can actually justify your views.
Or, better yet, complain about original movies and books. They're just being created, so there is no original "source material" to stay true to. Is there any particular reason why there can't be more of a diversity in cast? Is there anything necessitating your choice to write a male lead, or could they just as easily be a female? Would it make your story richer if the best friend is a different "ethnicity" (and I use the term in its most loose sense, because where we start creating hard classifications is where we mess up)?
Blame the audience--because in the end, Hollywood's just going to make movies that sell, and the TV networks are going to try to grab views. Is there a reason your favorite comedy or sci-fi drama has to have an entirely white cast? Why not seek out some less well known ones that have more of this diversity you want to watch?
(Spoilers: Some of my favorite series are Babylon 5 and the 1990's Flash. Because they did a good job with a diverse cast and made you not even notice. It was normal. There's an African-American doctor who I legitimately forgot was African-American because it wasn't as important a part to his character as his arc was. There's a Flash episode where there's a superhero who's African-American, and they don't go out of their way to point out HEY LOOKIE HERE WE GOT AN AFRICAN-AMERICAN SUPERHERO IN OUR CAST AREN'T WE FABULOUS it was just like "Oh, hey, this isn't weird and man this story's cool!" The best diversity happens when you don't even notice because it isn't pointed out, it's just treated as normal, and that's how life should be. We shouldn't notice these things. We should get to the point where we don't have to watch a movie without having to look for all the white actors. We should just appreciate a movie (or hate it, whatever) for its storytelling, its cinematics, the actor/actress's ability to act (or not act). The point where social justice will have succeeded is when we don't need to have these discussions anymore because it. doesn't. matter.)