Others in this thread are close to the truth when they say this is a light novel thing more than a manga thing. The reality is that this is a web novel thing, even before these stories become published light novels. I am going to quote an answer I wrote previously about this:

Long, specific titles have always been somewhat of a thing. For example, did you know that the novel we typically refer to as Robinson Crusoe, published in 1719, was originally titled thusly:

"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. Written by Himself."

In modern light novels, the long titles come from the ease with which amateur writers can self publish to a website like syosetu. This causes tons and tons of stories within the same genre or content tags to be listed alongside each other. So, having a long, specific title helps your writing stand out by acting as a quick synopsis rather than having a website visitor click through and read the synopsis of hundreds of search results.

A lot of our modern light novels were originally self published as webnovels to these sites and have therefore inherited the long titles that helped them stand out on those sites. I just wish that the titles would be adapted and shortened as well as they transition from webnovel -> light novel -> manga -> anime. Some of them just keep the long title but pick up shorter nicknames since the Japanese language is so prone to portmanteaus (konosuba, danmachi, arifureta, etc.).