Why? If this reality is (something like) a dream, it wouldn't be all that strange if the dream contents (drugs, brains) behaved in ways commensurate with what's going on in "reality" (the mind of God or whatever), without those dream contents being the cause.
I thought the same thing until I had my first experience. Now it's hard to maintain a materialist point of view...
I had the exact opposite experience. I’m having a hard time following the chain of logic that goes from a material substance altering the way you think to not believing that the mind is generated by the physical substance of the brain.