The author is a web novelist, so more than likely their stories were explored more and/or better in the WN. Unfortunately (or fortunately depending on your opinion on possible tearjerkers), it seems their stories were cut and butchered for the manga adaptation.I'm not sure why the author decided to add in the backstories of the deceased adventurers, since the MC loss of humanity didn't come up at all in the chapter. Not a whole lot of reason to raise empathy for those characters if the audience POV (that is, MC) doesn't have or isn't willing to explore it. Perhaps that will change with the upcoming meeting with his old partymate. 🤷
This kind of thing happens a lot in adaptations and for several reasons such as length restrictions, plot relevance, etc.