Hey, people in Taiwan are still greatly polytheistic today; the development of a better understanding of nature hasn't fazed them. And I imagine a lot of people are also like that in India (although I've heard the type of Hinduism that is vaguely monotheistic or plays down divinity in a tolerant way of atheism/agnosticism is really taking hold over there, with polytheists dwindling, but I don't have good information on that).I was unfamiliar with Summanus. A specifically nocturnal thunder god? Apparently, the ancients felt the need to deify every natural phenomenon, and were loath to rest until they had left no stone unturned; I wonder who the god for "the urge to poop" was. With that as the apparent imperative of polytheism, it seems no surprise that monotheism, which equally defies rational thought, would be so readily embraced by so many in the post- classical period.
I don't think the Taiwanese have a god for the need to poop, but there is indeed a number of toilet goddesses.
(The English Wikipedia article talks about Japan and historical China rather, plus a bit on ancient Rome. Not that the Japanese believe in gods much anymore.)