I’m not sure this is necessarily that case or at least definitely not entirely. Look, for example, at the gut and more specifically the neurotransmitter serotonin. It certainly is not produced based upon what is going on in the brain and they do not necessarily work in unison. In fact, the gut can even be more powerful than the brain in its production of various neurotransmitters, in some circumstances.
This isn’t to say the gut is now the key to performance or anything like that, but just to show that there are other ways to look at the situation and focusing squarely on the brain/CNS as the root of all things is at least incomplete, if not incorrect.