Okay so the story of how this trend happened is actually pretty funny.
Basically over in Japan, light novels have been a really big thing for a while now in a way. Short novels aimed at teens and adults with an anime or Visual Novel sort of tone to the writing and often illustrations took off there in print over a decade before they started being a mostly just online webnovel thing in English speaking countries.
So imagine you're in a shop and looking at all these shelves packed with narrow spines of tons of these light novels looking for something to read. If one has an interesting title, you might pull it out to look at the cover art and blurb... You see where this is going? Some clever person realised that the easiest way to hook people was to have a title that describes the premise of the story like a blurb, so basically put a condensed version of the blurb on the spine. As soon as this resulted in better sales, everyone started copying that format.
The format is also pretty handy for web titles because potential readers need to be tempted to click/tap to even see the blurb on a site like Tapas, so need to be hooked by the cover art and title, and a lot of readers go for high concept series based around a big, simple trope setup. If it's effective, it gets replicated.
.....of course to me it always makes me think of how the titles of troll movies in Homestuck are just several paragraphs of description of the plot. 