So where did the old culture gods and goddesses come from? Us. Literally, apotheosis. But they did/do exist, and are higher on the "spiritual foodchain". However they are not the Creator, the Source, the Maker of All. No, not the same as an old man with a beard in the sky, though one can certainly visualize God that way if that is your preference. The Creator/God/Most High has no form, is beyond understanding and form. Not so the Old Gods and Goddesses, though they are beyond merely human.
Leitch does a great job of describing this human to god transformation. They were all humans who died and who, through generations of ancestor worship, and through domination of weaker social groups by stronger, the stronger social group family spirits became gods, and then Gods. But notice: there still exists the supernatural. These are indeed real spirits, not just aspects of the mind. Solipsism and the belief that magic is simply psychological is ignorance.
Leitch does not talk about the other source of gods: Nature itself. The Thunder, the Sun, the Moon, the Wind, the Earth. And how some of these were personalized/humanized by people. This was the other path taken by humanity, especially those who retained the older hunter-gatherer path. Agricultural/sedentary peoples are associated more with ancestor-worship (amalgamated with natural elements) while hunter/gatherers more with nature-worship, especially in regards to animal and nature spirits. This is my observation based on data in the HRAF (Human Resource Area Files).
The gods and goddesses of human origin (the dead ancestors) did later go through human mythic synthesis with some elements of Nature. Thus Shango the ancient Yoruba king was worshipped as a powerful ancestor and then mythically joined with Thunder in West Africa.
Thor, the ancient ancestor-farmer-hero of the Norse, originally a human hero of a strong social group which would become the Aesir, became joined mythically with the Thunder in Scandinavia. Later the Aesir ancestor-deities would battle with the Vanir ancestor-deities, and become dominant, yet the more-"nature"-focused Vanir were strong enough not to be merely absorbed, but be an almost equal group with the more-"culture"-focused dominant Aesir...reflecting the human concern with the split between "culture" and "nature" (no it did not originate with Descartes!)
It was rare to have Gods and Goddesses in this kind of mode among most of the Native American tribes though. Most were of the hunter-gatherer mythic type, that is, there was less "human-god" types and more "natural-force" types. Among the Ioway, there was not a "god of thunder" or "goddess of fertility," but simply the Thunder (which sometimes could be seen in the form of a giant bird) or the Earth (which one sees in the form of an old lady in some of the mythic stories); Rabbit the culture hero is really a Rabbit and Coyote the antihero/trickster is really a Coyote.
I do know that the great Native American civilizations of the south did have "human-gods" like Quetzalcoatl and Tezcatlipoca of the Aztecs. That seems to align with Leitch's theory that city-state civilizations evolve those sorts of deities through agricultural sedentism, growth of population, and the subsequent domination of one clan's ancestors through the ancestors of a stronger clan, and then the increasing elevation of the strong clan ancestor-gods to those of National Gods and Goddesses. This certainly helps explains why there are so many gods in Central and South America, and the multitudes of gods among the Hindus.
But do not take this as "explaining away" the gods and goddesses. This is only an examination of their origins as human beings from the most ancient times. It is not saying they are not real, that they don't have an independent existence apart from us, or that they are not more vastly powerful than we are. The Gods and Goddesses of Antiquity are real as we are...because they WERE us, after all.