Why do so many cultures come up with similar divinities, even if they are at the ends of the earth from one another? This is something which explains the rise of all the theories concerning a group of people from, say, Atlantis, coming out of the sea and teaching various cultures around the world about their own mythologies and Gods at around the same time – this theory (and others like it) give people an easy explanation for the similarities.
There are two main theories for why divinities are similar across cultures, which are not entirely mutually exclusive. One possible answer is that the cultures with the most similarities are derived from one another (such as the various Celtic religions across Western Europe, or the Germanic\Norse Gods of Scandinavia and mid- to Eastern Europe), meaning that while the cultures will be different owing to being slightly different cultures which have arisen in different places, Another possible answer is that they are the same or similar because, while the cultures which have created divinities are all different to a greater or lesser extent, they are all reacting to the same phenomena.
The first theory – that cultures have similar divinities because they are related to a greater or lesser extent – is one which can only work in a limited number of cases. The Celtic divinities which can be found in the different parts of the UK, and France, respectively, have very similar divinities because the cultures are all connected with each other through the migration of the Celtic tribes which happened many thousands of years ago. This also holds true for the Germanic and Gothic tribes further into Europe – migration meant that their proximity to one another led to cultural exchange. While the different tribes in both cases had different names for their Gods, it was in response to the differences in their lifestyles and cultures, not because the Gods themselves were different.
The second theory, that of similar divinities being created in response to similar phenomena and events, is one which normally occurs to help us explain the basic archetypes of Gods which occur even in cultures which are completely unrelated. The idea behind this theory is that, since Gods and spirituality were, in many ways, answers and reasons that people came up with to explain why the world was the way it was, then they would naturally create Gods and stories which were similar to one another.
Think of the flood story, and how it seems to appear in a huge number of mythologies from around the world – there is the Biblical version, which many people in the West know, but it also turns up in Mesopotamian mythology, as well as Norse mythology, in various forms. The argument that this theory puts forward is that people did not have a scientific explanation for why crops grew at some points and not others, or why there was thunder, and so they created Gods such as Demeter or Thor. They then gave these Gods personalities and backstories as a way of explaining what was happening in the world around them.
We can’t be entirely sure why different cultures come up with similar divinities, though there are a number of different theories around, which range from the fantastic to the more credible. The theory you choose to believe will depend on how you see mythology.