I agree with C.S. Lewis that paganism is the default spiritual state for thoughtful humans.
Reason plus human nature = paganism
But clearly modern Western man is not pagan. Why?
The answer (very briefly) is that intellectuals are deficient in human nature.
Probably for evolutionary reasons - to do with the rapid evolution of higher general intelligence in high latitude large scale agricultural societies -
the most intelligent people...
(especially - perhaps - the most intelligent women -
...had the greatest reproductive success in these societies.
This created an intellectual elite which is deficient in 'common sense' and with an over-strong tendency to favour abstraction.
For intellectuals 'human nature' is much weaker; and there is a strong tendency towards denial of the soul, denial of transcendental goods; towards materialism, this-worldism, agnosticism and atheism.
When this intellectual class became the ruling class ('meritocracy'), they progressively imposed their atheism on the rest of the population via the mass media - initially via newspapers, magazines, books and lectures and later by the modern mass media.
That is why we are not pagans.
Remove the mass media, or remove the intellectual ruling class - and we revert to paganism.
At least for a while - but (all else being equal), monotheism will sooner or later overcome paganism (by its potentially superior social organization, perhaps; by its higher chosen fertility more certainly).