My band has the Avengers theme as a pre-game piece and I could not for the life of me remember how it went without grabbing my book and looking at the music.
Character themes (technically known as leitmotifs) are so important to filmmaking because they help your audience form connections between characters, which helps them engage with the story better. A viewer watching, say, a Harry Potter movie, who has never read one of the books before, might still be able to predict Voldemort coming into the scene by recognizing his motif, even if they don’t consciously associate with him. In fact, Voldemort’s motif directly paints him as Harry’s greatest enemy; it’s melody is just Harry’s theme backwards and in minor.
Meanwhile, Marvel relies on well known songs to carry their largest scenes. Take Thor: Ragnarok. The large fight scene at the end is put over Led Zeppelin’s Immigrant Song, and it’s the most memorable scene in the film. In fact, it’s the only scene I can picture directly. The rest? Disengaging and not memorable at all, because the music is meaningless background noise meant to just fill space instead of connecting us to the film.
Tl;dr: leitmotifs are really cool and Marvel’s refusal to use them probably lends a lot to why their movies are so disengaging.