Do you have any reason to suspect this isn't how the brain works? Maybe language isn't a small set of high level rules. Why should we suspect it to be? The probablistic models seem to be very similar to how real people actually learn informal language. Formal languages of course have high-level rules, and these are well modelled algorithmically.
I don't particularly have any reason to believe one way or the other. Certainly, the probabilistic models for language are created "out of the blue" without any attempt to model how human learn languages.
