Because once upon a time it did freak me out. It deeply disturbed me that the levels of chemicals in my brain could have an effect on my perceptions and feelings. What could it mean except that, yes, all we are is chemical soup, and the sense we have that there's a self there, a little guy swimming in the soup, is delusion.
Often one's first inclination on starting to think about "spiritual" things was that the spiritual world ought to be completely separate from the physical world. But the picture the Bible paints is of human beings as bodysouls, naturally occupying space and time, made as matter and spirit. It's not the case that our bodies are afterthoughts, simply (as Anne Lamott says) little earth suits we put on to clothe our souls, which will be discarded when we return to our original state of innocent nakedness.
Rather, the Judeo-Christian understanding of humans is that they were, in the original intention of the Author, a twofer. We are body-souls. And though the two can certainly be separated-- and at death in fact are, at least temporarily-- this state of affairs has always about it the reek of unnaturalness. Like death itself, this separation is not plan A, and will eventually be corrected.