Narration is boring. Montages have the potential to overstay their welcome. Exposition in dialogue is dumb. There’s already so much going on in the movies that adding more set pieces would actually generate the opposite effect. Busy movies feel like they rush and a lot happens in a short span of time (think what if tom bombadil). The only way was to actually cut more stuff to focus even more narrowly on fewer plot points, to gain time where to insert set pieces that illustrated the time passing, with slower pace. When a movie has very few things going on in a long time span, it feels like it’s illustrating a very long span of time. This is a balancing act that all screenwriters and directors have to face. For example, look at interstellar vs. Castaway, which one objectively is about a longer period time, which one actually leaves you feeling like the characters experienced a lot of time?
They already did all the incluing exposition they could. Only infodumping was left to do (the examples I gave). They actually did infodump at the intro of the first movie. They could’ve cut more plot points. But people would’ve complained it wasn’t loyal to the book even more, as they did at the time. Unless you turn it into a dozens of episodes over 9 seasons series, you won’t have time to convey the passing of time. Then you run the risk of it being boring. What we got was already a miracle. Look at what they did with The Hobbit, they butchered it for exposition.