I believe there are several problems with dialog intelligibility in film. The desire to play up the sound effects e.g. explosions, cars, trains, helicopters, gunshots and ambient sound for a "realistic" soundscape and to entice young viewers.
Reliance on relatively loud music to to tell the viewer how they're supposed to feel
When mixing the sound of a film, the producers and director, of course, know the dialog because they've lived with the film for a year or more. Over the weeks it takes to mix the sound the sound mixers end up memorizing the dialog so everyone involved can "hear" the dialog.

