If someone had been able to give me a clear - and believable - answer to your extremely important question, I would not have spent the subsequent four decades attempting to understand all the nuanced insights embedded in our inability to understand this issue. We can attempt to understand what we know and the vast number of things we don't know with psychological and historical/folkloric paths.

First, we can assert that all cultures have traditions. Three decades ago, I would have written "oral" traditions, but with the internet, much of our commonly-held, "popular" (i.e. folk), traditions are transmitted via the internet, so the media can change, but the fundamentals are still there. Also, since one of the first thing writing does in virtually any culture is to document the stories that were being told orally in each society, we know that the "myths, legends, and folklore" of your question have prehistoric/pre-literary roots, so the primal origin of that aspect of cultures can only be approached through speculation. Throughout, I'll offer excerpts from my Introduction to Folklore, which I used for my classes over three decades of teaching this subject. Here is some text on how fundamental folklore is to our humanity:

In all, the various forms of contemporary folklore are reassuring. They clearly indicate that folklore is alive and well in the modern environment. Disruptive changes over the past two hundred years have extinguished lifestyles and vast amounts of oral tradition, customs, and beliefs, but the folk are tenacious. People will create folklore, no matter the circumstance.

American psychologist Walter Fisher (b. 1931) has proposed with his “narrative paradigm” that people tell stories because the device is essential to human communication. He suggests that this aspect of our species is sufficiently significant that we should be referred to as “Homo narrans.” The degree to which this idea is valid may be open to debate, but the observation – that storytelling is indispensable to people – is at the heart of why there will always be folklore.

The field of psychology has provided one means to speculate on the origin of this body of story and belief, but no matter how enticing the various avenues of speculation may be, they remain speculation that cannot be verified. In fact, they are usually so far removed from anything that can be dealt with in analytical terms that one must approach that line of thought with more faith than deduction. That doesn't make the work of Freud, Jung, or Campbell wrong; it merely makes it difficult to address or evaluate when it comes to the subject of the origin of folklore and its various aspects. I'll add a separate reply on the psychological avenue to folklore.

Then there is the matter of what the fields of folklore and history offer in this context. Beginning with the Brothers Grimm, who published in the first half of the nineteenth century, folklorists were consumed with the question of origins. They recognized that folktales and legends (reflected in many Classical myths) fell into "types" - repeated plots that varied over time and space, but nevertheless "held together" with a core of repeated plot devices (i.e. "motifs"). This seemed to imply that these types had survived for centuries or even for millennia, but regardless of how long they lived, the question of a point of origin was either recent or pushed back in time. Either way, that point of origin remained elusive. Finnish folklorists advocated a systematic means of analysis that could answer the questions related to diffusion, change, and point of origin with a method that came to known as the Finnish Historic Geographic Method. This approach has come into criticism, but it was and remains the best means to attempt to answer your question. Criticism takes the form of those who say that even this exact method does little to explain the where and especially the "how" these stories came to exist. Other critics suggest that the "type" is an illusion and that stories are not as traditional as they seem. I take up this last criticism in my recently published The Folklore of Cornwall: The Oral Tradition of a Celtic Nation (2018) and we can discuss this further, but in general, I believe we can set aside this second concern. The first criticism, however, remains valid: we remain at some imagined "first telling" of a story, and that moment remains extremely difficult to fathom.

With advent of modern "urban legends" some folklorists hoped they had an opportunity to track down the "first telling" of these modern stories, which seemed to flash into existence and often lived short lives. Here, again, from my Introduction to Folklore:

The answer to this question is somewhere between "we don't know" and "it's just what people do" – that is, they 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 stories originate. For the most part, they were unable to find the point/person of origin to any general degree of satisfaction. It is a maddening question, and 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 might 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. Folklorists often asked renowned storytellers to tell a new story, to invent something that they had not heard. Without exception, these requests were refused by the storytellers who explained that while they repeated stories, they did not invent them. In general, it may be best to consider the possibility that the process of repetition and transmission is the source of much of the creativity that is evidenced in the international library of oral tradition.

One example may help shed light on the creative process: the Cornish droll tellers were known for their wild adaptation of the material they heard. Their counterparts in Ireland were the seanchaithe, professional storytellers who prided themselves in the faithful recitation of stories they had heard, as close to the original as possible. 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.

There is more to add - and I will - but this first post can start the discussion.