If neurophilosophy is a legitimate from of philosophy why don’t we have philosophies pertaining to various other sub-personal processes/systems: autonomic, vestibular, and proprioceptive? Why not Gastrointestinosophy? Isn't that the job of the empirical and experimental sciences?
A large portion of philosophy is concerned with knowledge or experience or mind, and has been for millenia. It's not that philosophy is invading biology, really; it's that biologists have convincingly demonstrated that the brain is what physically implements the mind--at least convincingly enough so that it seems prudent to re-examine philosophical thought in light of all the new findings about brain (and mind and neurons).
In particular, there are a lot of old thoughts that seem incredibly stupid (piles of false dichotomies and incorrect assumptions), as well as reasoning that now seems remarkably prescient.
There is no gastrointestiosophy because the gut did not turn out to be the material seat of consciousness.
I hope you don't mind me saying this, but you're question made me laugh. Gastrointestiphilosophy - now there's a course I want to take!
But seriously, branches and sub-branches of philosophy aren't 'created' by a board of academic philosophers, and only then can everyone write papers about it and get published. The order of events is the opposite: people were asking questions, and eventually enough material accumulated on a particular subject that it warranted getting it's own tag, for practical reasons. For example, there was a time (in theory at least) that if you took a course on ethics, you could comprehensively all the important ideas that were being discussed at the time, until lots of people had many more ideas, and applied it to more categories, etc. so that today we have metaethics, normative ethics, etc. and branches of applied ethics, one of which is bioethics, and today there's even enough material on neuroethics. Neurophilosophy is a branch of philosophy because what we've learned about the nervous system is very relevant to certain philosophical questions, making it a subject that has given philosophers much to talk about.
So the answer to your question is merely that there isn't enough philosophical material or questions about the lymphatic system and its philosophical implications as there are on the nervous system. But by all means, if you have philosophical questions about the lymphatic system, go ahead and ask them - talk to philosophers, get a discussion going, and maybe the day will come where I can take a class on gastrointestiphilosophy.