Why do light novels and manga have long titles?
The Anime Man explains this in his video.

TL;DR:


People get lazy to read the blurb of the novel (the short summary usually at the back of the book) to know what it's about, so a longer title will be more descriptive than titles like "Wish".
The LN industry is getting bigger, therefore everyone will eventually run out of titles to use (even movies there are some that have the same title which is not good), a longer title will make your LN unique and stand-out. Top selling LN in the first half of 2019 according to Oricon.