Jung’s collective unconscious is heavily misunderstood. It’s not quackery; it’s based on the assertion that there are biological or memetic imperatives that have been passed down from generation to generation, and also parts of the unconscious mind that function in a way that are common between people.
One can argue about Jung’s implementation, especially about whether or not the archetypes he identified are accurate and meaningful, but there seems to be a very concrete provable fact here: the psychology of people seems to bear commonalities, even in what would be considered extreme outliers.
Now, whether you want to argue about the more specific cases, like those of mythological figures appearing in unconnected contexts, Jung’s notion of synchronicity (mutual meanings, but diverse causes) is important as well: if dragons appear in mythology around the world, there does not need to be a real dragon or a social connection for those things to form. Instead, those can be independent functions of the way that people perceive the world and form a conception of the unknown.
Think of the collective unconscious as this: if you put three people in a white featureless room with a red circle painted on one wall, they will all see the red circle and Jung would argue that they all perceive the same thing.
The value they derive from that circle comes from the conscious mind. One person might consider it an eyesore, one might think that it has deep symbolic meaning, and one might fear the unknown entity.
However, they might have associations with the red that are common. If it were a crimson shade, it might evoke the effect of blood.
Thus, the collective unconscious may have deep and complex elements as Jung proposed, but it almost certainly exists at least in a form as a consequence of the brain’s physiology and common formative experiences with universal human concepts (like the risk of injury).
Disclaimer: I’ve read Jung, but I’m not a master of his work. This is sort of a rambling trying to make sense of his work rather than a masterful explication of it.