In pretty much every culture (other than the modern industrialized Western civilization), people have treated the existence of supernatural entities not only as a philosophical explanation for the origin of the world, but as an actual, undisputable fact of daily life. Not only might you theoretically meet spirits, many people actually did, and nobody thought there was anything too unusual about this.
Probably the main reason the Western culture no longer recognizes the existence of spirits is because Christian missionaries and inquisitors spent centuries burning books, executing shamans and witches, and generally systematically wiping out all knowledge of any spirituality that competed with the Christian one, through what was essentially a brutal reign of terror. And they pretty much succeeded.
Modern science is in many ways a pushback against Christian theocracy, and have proven very useful in the fight against the worst effects of religion, but even then, there are places where science still chooses to remain quiet. I think it’s possible that a big reason science hasn’t shown spirits to be real is not necessarily because they can’t be shown to be real, but because attempting to show that they are, in a world where authoritarian religion is still strong, probably would unleash all kinds of hell. (The spirits themselves might also realize this and limit their exposure to scientists.) In this view, believing that spirits aren’t real, simply because we live in a culture that doesn’t recognize them, seems to be an instance of chronological snobbery. Such a view not only ignores the massive amounts of evidence (even if mostly “anecdotal”) that spirits and ghosts do seem to have observable effects on the world, but it also seems to work quite hard at intentionally ignoring, avoiding, and explaining away all this evidence, using various explanations that are sometimes more fantastic than what they’re trying to disprove.
I’ve always been a true skeptic, meaning I’ve not only been skeptical of claims by people who I disagree with, but also skeptical of claims by people who I agree with, as well as “established” truths of my own culture. I’ve even been skeptical of myself and my own beliefs. That is what science is all about. I don’t believe what people tell me to believe, or that people around me believe. I’m a truth seeker — I’ll believe what the evidence shows, whether I like the conclusion or not.
Thing is, even though the Christians did wipe out almost all Western knowledge of spirits, the spirits themselves don’t seem to have gone away, they’re just much more careful about how they do things now. The evidence is there if you look, and it has recently convinced me to try to understand more about what’s going on. Not because I want to believe in spirits, but because I follow the evidence. (In fact, I’ve been a non-believer most of my life, and perfectly happy with that, but recent evidence has made me reconsider.)
An interesting thing about shamanism is that it’s the spiritual equivalent of science: there’s no dogma, there’s no blind faith, there’s only what you see and experience for yourself, and you can compare your experiences with the experiences of other shamans to figure out how things work. Shamanism is not a religion, it’s a method, just like science is. Thus, it’s a very powerful tool to find the truth, and even the origin of all religion. (And that probably also explains why established religions still fight shamanism so hard: it potentially exposes the real truth, not the limited truth that the religions want you to see.)
And, just as interesting, once you understand how things really work, and have found spirits willing to help you, you may find yourself able to affect reality in ways that traditional science still cannot. For example, to heal others in strange and wonderful ways. I’m still very much astounded myself every time it works and does something that shouldn’t be possible.