Of course you're right. It would be nice to think it to be one, though. My money is on legit crazy hermit/monk making his own language and writing out the things he sees in his crazy brain - Visions, distortions of reality, dreams, the like. If that's the case, even writing with his own alphabet in a new language, that language is likely going to be heavily influenced by the languages he knows - As a 15th century Italian monk (i say monk because they're the person most likely to know how to read and write), that would be Italian and Latin, with unlikely but possibly a bit of Greek or Hebrew thrown in.
mmm, outsider art generally doesn't group like that. There's a consistency to the Voynich that looks like consistent language and the ramblings of the madman generally don't. I think it was someone good at forgery and cryptography coming up with something to sell a rich rube. Who the forger and who the purchaser was I have no idea. There are several choices.