In his thought-provoking book "Why Materialism is Baloney," Kastrup speculates about why past civilizations have so often embraced a non-materialist outlook on life. Specifically, he suggests (as I understand him) that harsh conditions might have affected their brains in such a way as to impair local awareness, thereby giving them, by default, a greater access to the universal mind, or rather non-local consciousness.
While this may have been so, I'd like to propose another reason why past civilizations might have embraced a non-materialist outlook as a matter of course, and that is the fact that, to them, there were presumably no such things as "illegal plants," psychoactive or otherwise. They simply used those plants (and perhaps fungi) that were found to "cure their ills."
In the Western world, to the contrary (at least since opium was made illegal in the states in 1914) our governments have sought to prevent citizens from using plants that might give rise to greater awareness, plants that might otherwise link us with the vast unitary unconsciousness (or at least give us the intuition that such a realm exists). Yet when these "primitive" peoples were surrounded by the great pharmacopoeia of Mother Nature, they freely made use of ALL efficacious substances, without first deciding that some were off-limits because they tended to produce transcendence in the user. To the contrary, transcendence was generally seen as a great gift from the gods and part and parcel of why a given substance was efficacious in the first place. If substances were placed off-limits, it was because they were so holy in their transcendent effect that they had to be administered only by priests or priestesses (as in the Eleusinian Mysteries).
Thus everyday medicinal plant use exposed the people to transcendence without fanfare -- in a way that modern Western governments no longer allow.
To sum up (and elaborate) by rewording the initial question a little bit:
Q: Why don't Western civilizations observe the transcendent realm that was so obvious to most "primitive" civilizations?
A: Because law-and-order politicians have outlawed transcendence by outlawing the natural substances that unequivocally provide it.
It must be noted that the politicians have succeeded in doing this partly by appealing to the mindset of scientific materialism. After all, if scientific materialism is right, then there's no such thing as transcendence anyway, hence the so-called "medicinal" use of natural psychoactive substances does nothing but produce crazy hallucinations. So the drug war makes strange bedfellows: both materialists and right-wingers disparage those medicines whose mechanism of operation involves inducing transcendence in the user. Of course, their first step in disparaging these medicines is to saddle them with the pejorative label of "drugs."