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I do wood scuptures and I have to finish the entire piece if I want it to stabilize. the idea is to seal in what ever moisture is there so it can even out and not cause stress (which causes cracks). and to keep extra moisture from being absorbed.

So why isnt there a thin layer of a moisture barrier on the inside of an acoustic guitar?
I expect the answer is to allow the wood to resonate. If that is true then why would you use a filler on the pores on the outside when finishing?

the guitar needs to breath but there are examples of inside finish. I don't know any builders that do. If you do finish the inside it makes future repairs difficult.

John Bogdanovich finishes the insides of his guitars with shellac.