Culture changes around you; it's a lot like the old frog in that pot of slowly boiling water. You don't even notice that the word you thought was the correct term for another race/creed/color/sexuality is hate speech until someone yells at you for it. The definitions shift as power moves away - slowly, ever so slowly - from centralized white maleness. Some of that stuff is extreme - go to Tumblr and look at the made-up pronouns people use to refer to themselves because they believe they're wolves or clouds or something - but that extremity helps pull the mainstream along to the next level of awareness and decency.
Look at movies from the 80s; the racial and gender and gay stuff in a lot of mainstream movies from that era are downright shocking today (I'd argue even more shocking than some of the racial stuff in movies from the 30s and 40s, which at least are periods that exist Pre-Civil Rights Movement*), and those attitude changes happened slowly and almost without us noticing. Are we ever going to get to a point where Weird Science is as subtly suppressed as the hyper-racist Little Rascals episodes?
We're in the middle of another one of these attitude shifts, and it's about representation on screen. More and more people are realizing that women and people of color are simply underrepresented on cinema screens. One of the reasons that the Fast and Furious films have such a huge fanbase is because of its quiet diversity of casting - all sorts of people are represented in these movies. And they don't make a big deal out of it, they just hire people of different races (and have female characters) and go about making lots of money. Let's put it this way: when Fast & Furious 7 lost Paul Walker the large ensemble cast lost half of its white male good guys.
That's one part of making our movies look like our world - making it so that 'white' isn't the default color for lead characters. But there's another, weirder thing going on, and it's called whitewashing. It's when a character who should be non-white is either made white for the movie or, worse, played by a white person in some kind of make-up. Pretty much every old movie with Asian or Native American characters did this; John Wayne played fucking GENGHIS KHAN at one point.
I want to take a quick aside here to address a point that will, without fail, be made in the comments: yes, the skin tone of an actor should be as important to their casting as their hair color. Yes, performance capture technology is bringing us to a future where any actor can play any race, gender, age or species thanks to the magic of technology. But that's utopian stuff, things to consider once our society has stopped getting broken down along tribalistic lines, lines often demarcated by skin color or place of origin. We don't live in that world, and that means we have to be vigilant to make sure that the people we're seeing on film reflect the people sitting in the audience.
And one more side note: when you look at a casting landscape where 'white' is the default setting for casting and you say 'It should go to the best actor,' what you're kind of saying is that the best actors are white. Because if you think the roles are going to the best actors, and all the roles are going to whites, you're basically claiming that there simply aren't as many blacks or hispanics who are capable of the quality of work that whites can do**.
Anyway, back to the main point, which is whitewashing. It still happens - look at Jake Gyllenhall in Prince of Persia or the leads in Avatar: The Last Airbender. And now it's about to happen on a fairly major scale, as Ridley Scott's Exodus: Gods and Kings will feature a very, very white cast playing likely non-white Ancient Egyptians.
I have to go on another side journey here - there is some controversy over what color the ancient Egyptians were. I'm going to go out on a limb and say 'Not the same color as Christian Bale, who is Welsh***," and the general consensus is two-fold: 1) the race of the Egyptians doesn't actually matter in terms of their culture and history because skin tone wasn't an issue in those days as it is now and 2) they were probably what we would call black or at the very least MIddle Eastern in appearance. They may have actually been a mixture of two peoples from different parts of Africa, even.
While race didn't matter at the time, it does now, and casting all white people as non-white people is problematic. Ridley Scott, a 76 year old man who grew up in a world where the historical epic tradition was actors with booming English accents wearing togas, probably never even thought about it. It likely never even crossed his mind that he might want to seek out people of color to play these people of color. He just stocked up on bronzer and cast Joel Edgerton in his movie instead.
Everybody else is going to notice, though. Cultural shifts happens quietly and slowly, like the movement of Earth's tectonic plates. All that quiet, slow movement results in something bigger, though, as the plates smash together and cause earthquakes, and I think Exodus: Gods and Kings is riding a tectonic plate that's just about to crash into increased social awareness of representation. Will the movie be good? Maybe. Probably! But will the movie cause controversy? I don't see how it won't.
And that controversy could be the one we need, the big central moment where everybody has a talk about this and says, "Hey, you know, maybe we shouldn't cast white guys as every race. And maybe we should figure out how to get people who aren't white guys into our movies." I really feel like Exodus is going to be one of those movies that crystalizes a larger cultural issue and makes it clear for lots and lots of people. There will still be those - white men, almost exclusively - who scoff at the idea that white people playing every race under the sun is a problem, but they're a vanishing breed.
Maybe, in the utopian future when someone makes a movie about the 2010s, a famous Asian actor can put on whiteface to play one of these guys.
* in our pop cultural memory, not in reality.
** It is important to note that the 1970s showed us that Italian actors are the actual best actors, and they should probably be cast in every role.
*** Christian Bale, by the way, isn't actually playing an Egyptian, he's playing Moses, a Jew from the Middle East who also would not have looked a damn thing like a Welshman, so the point remains.