When all of those Hollywood hacks and disillusioned writers-turned-screenwriters say that dialogue is all but unimportant, because it's changed by actors, directors, and can be changed by any non-creative wannabee who has any kind of power or influence, when these writers say that the important thing in scripts is plot, images, mise en scène, and character development (as if character development were a thing apart from dialogue), what they really mean is they can't write a film where dialogue is important, they don't have the influence, or they are not good enough to write obviously potent scripts (or they don't have the power to make the kind of film they want to). But a master of the form can do it: witness Martin Scorsese's Age of Innocence.
Novels can be translated more or less directly into film art. The problem is not in the art form, nor is it the filmmaking process that belittles the role of the writer; the problem is in the audience. If they are not literary, if they don't read books in the first place (which, let's face it, most of them don't), then how can we expect them to like a book on film? It's a minor form in a world where image is king. Yes, the image is the most important thing, but only because the audience makes it so. In cases like Age of Innocence or Washington Square image is less important, taking on a "more equal" place with dialogue. Literature can be translated onto film, but it isn't often done, because the audience is seldom up to it and, after all, although we're always talking about the backers, the true financiers of most films are the audiences.