3) They revel in moments
Narrative films depend heavily on plot progression, reaching certain pivotal points in a story at specific times in a movie, which requires this fairly constant forward momentum, a busy-ness that prohibits them from stopping and really milking individual moments for much time at all. Every now and then there will be a Ferris Bueller that puts exceptional value in stopping and looking around once in a while, as it were, but really dwelling in a scene or a moment usually gives a non-musical film the designations of “poetic” or “avant garde,” which for some reason are meant to be pejorative.
Lots of the more deliberate pacing of slow cinema takes advantage of the poetry of scenes and moments, but no other genre accomplishes this in the way musicals do. The entire premise of the show-stopping number, where the show or story literally stops so that characters can sing about what’s going on with them in a particular moment, depends on the interest in what’s been called the vertical development of character (or just spectacle), as opposed to the horizontal development of plot.
Again, this is a change of pace from most movies, which is also strange because while cinema is special in the way it can deliver precisely timed plot flow, it can also provide the most precise form of moving audio-visual expression. Musicals are able to harness that potential energy into scenes that allow viewers to stop and enjoy the sights and sounds of the spectacle before them.