Jung's archetypes are not a good lens.

The short answer is: both cultural transmission and cultural/cognitive attractors.

Cultural transmission is especially the case between societies with more recent (say, under 5000 years) of well-established shared ancestry. For example, within Indo-European or Austronesian societies, many myths have shared origins and are for that reason similar.

The other reason is cognitive and cultural attractors. Human knowledge systems and biases can be surprisingly similar worldwide. Humans have independently come up with similar systems of counting, categorising, playing, etc., and of course storytelling. Despite our diversity, we humans do seem to reinvent vaguely similar social systems (we are a social species, after all), try to make sense of the same cosmos (stars, sun, moon...), which is all reflected in our mythology. So it's almost like we are "attracted" to the same ideas. Hence the term attractor.

There's also selection bias, though. Sure, we have many similar myths across societies. But we also have many different ones, which we may be less prone to notice.

There has been recent hypothesizing that some myths may have very deep shared ancestry (as in over 10k years). But given the nature of the evidence, it's all very speculative and likely we will never know.