Your question contains "common elements" that are expressed repeatedly on this subreddit - all without any clear historical connection! The question simply arises independently and frequently, so we must consider some other reason as to why it frequently appears!

My typical answer to your question is as follows:

There are several ways to address apparent similarities. The first is the simplest explanation, namely that while oral traditions may seem similar, that can be deceptive. The human mind is inclined to see patterns. Seeing animals in the clouds or faces in linoleum does not mean that there are actually these entities (or their spirits) in those locations. We arrange the world so it makes sense, and seeing patterns - and similarities - is one way to achieve that. So first, similar-seeming traditions are not necessarily that similar nor are they necessarily connected in some way.

The second way to explain similarities is to consider the possibility of diffusion. Traditional narratives diffuse, and although they change over time and space, some core motifs can (and do) survive the process. This can explain some similarities: in this case, similarities are part of a historical process that involves diffusion. Descent from a common ancestral body of traditions explains why "cousins" have similar oral traditions; diffusion from one culturally-unrelated people to another explains how some similar stories can appear in diverse places.

A third possibility is less difficult to deal with and has yielded some extravagant contemplation. This is, namely, the idea of the common human denominator. Some of this is easy: all people die and experience death, so it is not surprising that all folklore deals with death in some capacity. Because most people historically have preferred to contemplate the idea of survival of death, it is not surprising that most oral traditions deal with ghosts, walking dead, etc. While similarities may not be profound in all cases, the common human denominator causes some similar attributes.

The same can be said for many other aspects of the human experience: sex, but also the weather, flooding, animals, etc. All people share a great deal in their experiences, so it is not surprising that internationally, oral traditions would be similar.

Taking the idea of the common human denominator a step further, Carl Gustav Jung postulated the idea of a collective unconscious - the idea that there is a shared body of archetypes that is entwined in everyone's consciousness, part of a universal fabric that manifests in our narratives and in our dreams, etc. This idea was transformed for more popular consumption by Joseph Campbell, but regardless of the promoter of the idea, the foundation of this way of thinking is the same. This is next to impossible to evaluate. It is nearly a spiritual explanation and it must be taken on faith - or not - depending on the person.