This is an assumption of mainstream materialism and neuroscience that may not actually be true. The Whitehead process philosophy and physics that I find useful assumes that mind is in fact causal, at every locus in the spatiotemporal hierarchy that is brain/mind. There is a lot of work on top-down causation or radial causation philosophy and neuroscience now, so you assumption of only bottom-up materialist causation is just one of many viable approaches today.