Obviously an insanely imperfect analogy, but kind of fun to noodle on, after having the initial thought actually in the shower. At the simplest level, do you need to cram multiple epic adventure tales, liberally dosed with didactic religious content, into a single human brain? Meter and repetition and tropes become your best friend. Beyond that though, there are still ways that poetic techniques pack more meaning into fewer words than prose, which gets described as "poetic" when it effectively does the same things.

If you find the right turn of phrase, the combination of sound, connotation, and (hopefully) shared cultural touchstones (""Darmok and Jalad at Tanagra"?) means you can describe an entire scene effectively without the multiple paragraphs otherwise needed to set out every morpheme of intended communication. Now, as pages of writing become cheaper and more accessible, they also take over the use cases where efficiency of communication was imposed rather than sought, but the toolbox remains there for those who simply like the exercise, or where there is still value, such as in verbal communication tied to a musical arrangement that needs to wrap things up before the audience loses interest. Also like compression, there are libraries that need to be installed and processing overhead involved to decompress the meaning that has been encoded into fewer words than strictly necessary.

Limitations to the analogy I'm already thinking of: Subtext exists regardless of how wordy you are. It might be a false dichotomy to think you can separate poetry from music at all.