Out of morbid curiosity, why do you arrive at the conclusion that the writing of a person should significantly depart from that of their regular speech patterns? I mean, to be candid, that actually is the case for me in a much different way that isn't relevant in this context and I don't care to discuss but you couldn't be more incorrect and inscrutably so with regard to this context.
If I'm being glib, I would say that's a sign of mental illness where there's not a code-switching economic/practical basis for doing so [hmm emoticon]