Because the whole point of the character of Stalker (and thus of the "Ends of the Earth" WW storyarch) is that a living, thinking, sentient being without a soul is doomed to a horrible existence, much worse than death and torture.
This isn't meta or fandom, this was said by Diana herself in the story where her soul was stolen, and she was forced to take it back by dooming the thief. Since the concept of soul had such a great and fundamental importance in the story, I would have liked it to be explained more in detail.