This all goes back a while, to about the time that Batman Begins was casting. Christian Bale as Batman didn't really get underniegh people's skin last I remember. The main focus was a new take on characters after Batman and Robin. They started to anounce casting, but kept one actor under wraps, Liam Neeson. Credited as Ducard, not Ra's al Ghul, there wasn't much to be said. However, can you imagine what MIGHT have happened if they didn't reveal the twist? The internet would have gone mad! An arabic charcter, being played by an Irish actor! However, he ended up doing a good job, becoming a very underrated villain. This was a risky choice, deviating from the source material like that. And Neeson wasn't even known as an action star back then. Warner Brothers didn't focus on looks or race to pick the actor, they chose an actor to potray a character with the traits defined by the comics but adapted for film!
Next we have Heath Ledger, who is the stereotypical "out of the box" argument. He didn't potray a character straight out of a comic, but a charcter in a movie based on a comic book character. And he wasn't well responded to as a casting choice, he really was ostracized. Now, he is considered to have given the best potrayal of a comic book charcter ever! And Michael Shannon wasn't really accepted as Zod. But again, this was proved wrong. Now we have people swearing up and down that Ben Affleck, Jesse Eisenberg, Gal Gadot, and Ezra Miller aren't right to potray the charcters Warner Brothers have casted them for.
Well here's the thing, and I know this may be shocking: Warners does NOT go out on the street and say "hey, you, do you want to be in our multi-million dollar picture?". They go through the lists, have actors audition, make offers, and choose actors based on how they want their charcters potrayed. Look at any fancast, and you would probably have seen none of these choices. Well, thats because Warners knows that fan's don't always know what they want. They go with who they think will potray their version of the character based on the comic. It worked with Liam Neeson, Heath Ledger, Michael Shannon, and others that I may have failed to mention, why would their process stop working now?
Moving to Marvel, they do deliver fan serivce. They have had their share of controversies, but most of the time they chose who they think fan's want. Their recent choices of Chadwick Boseman for Black Panther and Benedict Cumberbatch for Dr. Strange proves that they know who fans want. But, what if this does fail? Many don't think this is possible, but I assure you, it only takes one bad movie. Right now, Marvel can do no wrong I supposed. Until they take a risk. Until they say "hmmmm, maybe so and so isn't a great choice, maybe we should go with so/so" Can you imagine that?
The true difference between WB and Marvel casting is what they are filming for. Warner Brothers casts for a comic book MOVIE, while Marvel casts for a COMIC BOOK movie. Is either wrong? No. But what if, and I'm just shooting idea's right now, both actually have an idea for the future? What if WB isn't "trying to run their movies into the ground" and Marvel isn't "just feeding the fan's what they want so they don't get backlash". There are reasons for every bit, a method to their madness.