What's just as interesting is that local deities in these forms of animist deities tended to be based upon specific villages and cities. When tribes of multiple villages sprang up, polytheism occurred as a way to explain that actually, your local deity is a brother or sister of our local deity. Oh, but we conquered your village with our mighty city, so obviously, your village's god is a child compared to our daddy god.
For example, when there was a rivalry between Sparta and Athens, it was reflected in the conflict between their deities, Ares and Athena both contesting for the title of god(ess) of war, with both deities personifying the two cultures different view on how war should be enacted.
The Babylonian pantheon was even more strange, as Marduk was made the highest god of the pantheon in spite of not really being god of anything in the way that Zeus was god of the skies, but Marduk was god of Babylon (the city, not the empire), so therefore, he was top god.
Greco-Roman Gods would rise and fall out of the pantheon's top 12 based in large part upon which cities were the most economically powerful.
In fact, if you look at early Judaism, it is largely distinguished by being a local religion (YHWH was "god of the mountain") that had a strict "no other gods before me" clause that only eventually developed into monotheism.