Women (half the population, after all!) and minorities are expected to sympathize with the white male protagonist and his (usually also white, male) buddies. Sometimes the buddies are women; sometimes they're minorities. But it's never their stories, only Mr Whiteguy's.
Imagine there's a popular kids' TV show about a group of (pre-)teens who come from a world which is based extensively in East Asian cultures, like China, Tibet, and Korea. Two of the main characters are siblings, who are drawn with a medium brown skin tone and whose culture is clearly Inuit-derived. Another main character is clearly based off of Buddhist monks. One of the nations in this world wears topknots to denote rank. There is extensive use of Chinese martial arts in their world.
Imagine a famous director, himself of Southeast Asian origin, wants to make a movie of this, and the creators give him the go-ahead. Imagine that this director casts every main character, including the 2 who are clearly depicted as darker skinned, with white teenagers.
You don't have to imagine too hard. This is the proposed cast for M. Night Shyamalan's Avatar: The Last Airbender movie.
Change the scenario slightly, and you get the SciFi channel's whitewashing of brilliant SF writer Ursula LeGuin's Earthsea books. She purposefully wrote a fantasy where the main character was non-white and the villains are blond, blue-eyed white people. Then SciFi cast a white kid as the main character (and changed the entire story, but that's a rant for a different day.)
I didn't see why everybody in science fiction had to be a honky named Bob or Joe or Bill. I didn't see why everybody in heroic fantasy had to be white (and why all the leading women had "violet eyes").
But when stories with people who aren't "honkies named Joe" are made into movies, the casting is full of honkies. Because it's OK to make people who aren't white males watch movies about people who aren't like them - we're supposed to identify with this character, who is the default and ideal in modern American society - but it's impossible for white males to identify with characters who aren't white males.
There seems to be some sort of cognitive dissonance at work here. Oh, wait. That's privilege.
Edited to add: A fan has made a convenient how-to post for letting Paramount know your displeasure here.