Basically, most of these animes are light novel adaptations and the market for then in Japan is extremely saturated. So by making the title as descriptive as possible, there is a chance that it will catch someone's attention as opposed to having a nondescript title.
It happened because people figured out putting the whole synopsis in the title on the main LN/WN site in Japan gave them more clicks and then everyone started doing it.
It's basically the same as YouTubers having to put their face in thumbnails to get clicks.
"The Life and Strange Surprizing Adventures of Robinson Crusoe, Of York, Mariner: Who lived Eight and Twenty Years, all alone in an un-inhabited Island on the Coast of America, near the Mouth of the Great River of Oroonoque; Having been cast on Shore by Shipwreck, wherein all the Men perished but himself. With An Account how he was at last as strangely deliver'd by Pyrates."