This presumes that each utterance is independent of the others, and clearly violates what we know about how language and conversation works, as discussed in the next section.
there have been many proposals to inject information from a longer context into standard LM architectures, going back to Ngram models (Bellegarda, 2004), or to generalize N-grams LMs to operate across utterance boundaries and speakers (Ji and Bilmes, 2004).