The simple explanation for all of this is that western audiences regard Animation as a medium solely for consumption by children. Disney/Pixar will likely always produce animated films for children first compared to Anime which produces films for all audiences.
"The Kingdom of Dreams and Madness" [1] is a documentary worth watching if you've seen any Studio Ghibli films. The opinion of Miyazaki is that Animation is a more freeing medium than traditional film. It's available on Netflix right now for those who have access.