Bottomline... Appearances matter. Take for example the casting of Captain America.
They went with "Chris Evans", a muscular, good looking white dude. Because that is what Captain America looks like in the comics. If they would have gone with say "Michael B. Jordan", a muscular good looking black dude, it wouldn't have been accurate.
Plus it would have taken everyone out of the story being told on screen. Because they'd be thinking in their head "That's not what Captain America looks like. This isn't real!" The same can be said for Ironman, Thor, Batman, Superman, Harry Potter, James Bond, etc.