They don't, or any movie. But they will jump on anything as an excuse to grind their respective political axes, without caring who is damaged in the fallout.
Let's not get political here. Anytime people bring up "the Left" or "the Right" is never a good sign for the health of a thread.
I would have cited Benedict Cumberbatch as Khan or the Wall, where the main hero is Matt Damon. Chloe Bennett has also said that she was turned down for loads of work and bagged her first audition after changing her name to sound Caucasian.
Or
Gods of Egypt that had a predominantly white cast, even for the Egyptian gods. Or
Exodus: Gods and Kings.
The Wall wasn't as bad, but came at the wrong time. There were a couple other examples that year, so it became "the third in the list". Really,
The Wall is more an example of the white saviour, where a white dude has to come in and save the day. But, when watching the movie, that movie was much more of an ensemble piece with white and Chinese heroes. And it doesn't work as well without the outsider, as you need them to serve as the audience proxy when explaining the backstory.
Anyhoo, that's off topic.
I agree that you should cast actors based on their ability. And, as you say, with obvious exceptions for historical figures. Or real people and not just historical figures, because someone shouldn't have to be "historical" to be played by someone of the same ethnicity.
But there's the two other topics that smash into that. As OB1 says, there's
whitewashing, where you give a part to a white actor rather than a minority or person of colour. Typically casting a white actor as a non-white historical figure.
This wouldn't necessarily apply here for the drow anymore than it would apply for Klingons in Star Trek. The make-up erases the ethnicity and doesn't match to a real person. Kruge wasn't a real historical person so it's not an issue to have him played by a white actor. And since the Klingons were effectively stand-ins for Russians in the political narrative, it very much was not whitewashing...
The bigger issue is
blackface. Casting white actors and painting their face black. Which ties into a whole lot of minstrel culture and mocking of African Americans in theatres for the entertainment of whites, and in a modern sense also invokes cultural appropriation.
It's a messy thorny issue.
Now, you can cast white people as dark-skinned non-people. Again, see the Klingons for an example for how it can work without upsetting people. Because it's a very different culture and not associated with any minstrel tropes.
The third issue is casting black people as "the bad guy". This is the more problematic issue for the movie. Because fantasy tends to take place in a very whitewashed Eurocentric world. And thus the "good guys" tend to be Caucasian and the monstrous, savage villains tend to be dark skinned.
Dark as evil is a pretty old concept that doesn't really match to racism. Black being evil can be found worldwide. Light/ dark dualism is big in Zoroastrianism, which is from the Middle East region. Japan has the ying-yang. So presenting the bad guy as dark skinned (like the orcs in
Lord of the Rings) isn't intentionally racist.
Buuut...
When when the 20% of North American audiences watch the film and don't see anyone that looks like them as a "good guy" and the only people remotely like them are "monstrous bad guys" that makes them uneasy. Even if they're a unnatural dark grey colour (like the orcs). It feeds into a problematic narrative. Like how the only Middle Eastern characters in action movies are terrorists.
Again, you can have the bad guys as dark skinned without a problem, if there are also people of colour on the side of the angels.
Going for the Star Trek hat trick, it doesn't matter if the Klingons are evil and dark skinned. Because the
Enterprise has a diverse multiracial crew. Black people watching the show wouldn't see the Klingons and bad guys as the only people that looked like them.
I'm not 100% convinced that it's necessary to go the whole evil wizard has an army route again. The finale could be great with a big bad monster. Maybe I'm just nostalgic for Ray Harryhausen movies, or even Raiders of the Lost Ark...
The evil army works nicely because it allows an extended action sequence for the climax where the team of heroes all fight waves of mooks. A battle sequence with a group of people against one person requires a lot more coordination and choreographing.
But a big monster would be easier.
Really... the big bad of the movie
needs to be a dragon. The D&D movie absolutely needs a dungeon and dragon. And having a pitched epic battle against a dragon would be fun.