Let's take a different view of Myth and Legend. There may be a deeper reason why they survive.
It's only in fairly modern times that the majority of people can read and write. Historically such pastimes were the province of clerics. When the Aristocracy sent their children to be educated by the priests only then did they become literate. For the overwhelming majority of time that humans have been around very few have been able to read.
So how do you teach such people? How do you teach the best way and time to plant and harvest? How do you teach moral lessons? The answer lies in story, in myth and legend. Here's an example;
Sith the beautiful was the wife of Thor the God of Thunder. Her glory was her hair. It was as fair as fine gold and hung down her shoulders in a majestic cascade. Loki, the mischief maker, tricked Sith and by magic had all her hair shaved off. To hide her shame and embarrassment Sith donned a white cloak and pulled the hood up over her head. When Thor discovered what had happened he flew into a rage. He grabbed Loki by the throat and shook him. He promised that terrible things would happen if Loki did not make it right. So Loki went deep into the Earth and made a deal with the Dwarves. The Dwarves spun fine gold thread and Loki, again using his magic placed it on the head of Sith so now she was even more glorious and beautiful than before.
This is the tale of the wheel of the year. Sith is the Earth and Mother to us all. Her hair is Barley Corn, the staple diet for Northern Europe. In the autumn the corn is harvested and the Earth lies bare. Then comes the snow of winter covering the Earth in a white cloak. In the early spring there are storms and thunder then the Earth warms, and from deep in the ground magic happens. The corn again begins to grow and harvest time will soon be around again.
Such stories, told and re-told, kept our connection to the Earth and her seasons. Sometimes they held deep meanings and lessons not immediately apparent. For example the Tale of Lleu Llaw Gyffes from the collection of Welsh Myth known as the Mabinogion;
In this tale Lleu is wounded by a magic spear thrown by his wife's lover, Gronw. Transformed into an Eagle he flies away. His foster father, Gwydion, searches long and far. Finally he finds an old sow eating something at the foot of a great Oak tree. Looking high into the branches he sees, on the topmost branch, a great Eagle. The Eagle is wasting away with feathers and flesh falling from him. So Gwydion sings to the tree. In a song known as "Englynion Gwydion" he sings three verses. After the first verse the Eagle descends to the middle branch. After the second verse the Eagle dropped to the lowest branch. After the third verse the Eagle fell into Gwydion's arms. Then Gwydion spoke the word of power and the Eagle once again became Lleu.
In the Englynion is contained one of the secrets of Druid power. Each verse contains a commitment to truth, a declaration that if truth is not spoken harm will befall. Druids were known as men of truth and this absolute commitment to that principle made all believe that if a Druid said it would happen it would. The whole tale, among many other lessons stresses the need of honor and truth.
So entertaining stories provided our ancestors with needed information, entertaining stories also taught moral values. Lets look at another more controversial reason for their survival.
Suppose that the ancient Gods really did exist. If that is true then surely they would have been able to foresee the future. They would know that the God of the Hebrews and his Messiah would come to dominate the world and the knowledge of them and their ways would be lost.
So what if the Gods told stories, Myths and Legends that contained within them, not just the things mentioned above but the secrets of the ancient Gods and their magic. So what if the reason the myths endure is that they still have something to teach us that we have yet to fully uncover.
Look deep. There may still be hidden treasures.