Answer

Historically, this is probably a record of what actually happened in the study hall. Why they chose to leave it that way is not entirely clear. There are good explanations for why the Talmud does not operate in a top-down way—that is, by establishing rules from which details are derived—but instead prefers casuistry based on cases and analogies. Rigid thinking usually fails when dealing with complex things, and life is complex. It seems to me that the Talmud wants to teach a mode of engagement rather than concrete content, and for that an associative give-and-take, which shows how everything is connected to everything else, is very fitting.