It's probably easier to answer this question as it is "related to almost any religion or culture." The answer is somewhere between "we don't know" and "it's just what people do" - i.e. invent these sorts of stories.

Several folklorists, including the late Alan Dundes and Jan Harold Brunvand, recognized an opportunity with recent urban legends to finally track down "the source," the point of creation, to better understand how these thing originate. For the most part, they were unable to find the point/person of origin. It is a maddening question, but much like the proverbial chicken and the egg, it isn't likely to be resolved. People tell stories. People repeat stories. And as they repeat them, people modify stories, which sometimes results in the birth of new stories. There must have been some highly imaginative, creative people who invented stories at some point, but that sort of person has yet to be identified in the annals of folklore studies.

Coincidentally, I am doing research into the Cornish droll tellers, the counterparts of the professional storytellers of Ireland, the seanchaithe. While the Irish seanchaithe prided themselves in the faithful recitation of stories they had heard, as close to the original as possible, the Cornish droll tellers were known for their wild adaptation of the material they heard. Perhaps in this, we can see the opportunity for new stories to be born, and indeed, Cornish oral tradition includes more than its fair share of distinct subtypes (i.e. variants) of legends and folktales, perhaps offering a hint as to how these things may come into existence.