I never thought that you are a reductionist . Although there is some irony to your statement. The philosophy of mind is the only field where a reductionistic answer is even considered anymore, but even here it is only a handful of people (The Churchlands and Alex Rosenberg for example).
Side note: Especially Rosenberg is a very interesting character and although I don´t agree with anything he offers, since eliminative materialism (the denial of existence of mental states/these are illusions which have to be reduced to the mental states) is a self refuting position, he is nontheless a widely known philosopher and maybe, ironically, the key for the ultimate death blow of materialism. Why? Because if his arguments in his book “An Atheists Guide to Reality” can be defended as valid, then a materialist and naturalist consequentially has to adopt eliminative materialism.
Currently I´m reading “The philosophical foundation of neuroscience” by philosopher Peter Hacker and neuroscientist Mike Bennett. It is somewhat the 21st century version of Poppers and Eccles “The self and its brain”. And dare I say, it was desperately needed. Especially Hacker is a furious critic of the current “neurobabble” plaquing the consens and of those mentioned eliminative materialists. But to answer your question in a brief manner: It is because there is no objective way to read from your pattern of firing neurons what you are thinking. The same thoughts would make for a very different picture in your and my brain. But more importantly: There is an indeterminicy of meaning in the pattern. The firing neurons don´t reveal the associations made with the thought.
The problem now is that I wished to give you a longer aswer, but I can´t talk about added aspects like “aboutness”, the ability of thoughts to be ABOUT the thing, which needs explanation as to how a the aboutness comes about (hehe). So I would love to give a longer answer, but your small question is pretty much the whole field of the philosophy of mind. Interestingly, this made me think, that something like Plato´s forms have to exist kin order to account for the meaning, aboutness and intentionality in our thought processes.
Added note: Your medical examples are no examples to support the case for a materialistic case, because they are unproblematically compatibel with everey kind of dualism. Even Descartes expected a very tight correlation between the soul and the brain/material processes.