Or it could just be that normal human conversation tends to flow naturally from topic to topic in an stream-of-consciousness style, with one comment triggering an association with a new tangent of thought without formal divisons or pauses. Saying, "but back to what we were originally talking about..." seems to be the primary accepted way to refocus the conversation to the original topic in polite society. It requires little effort and is considered in most cultures to be a normal and necessary element in an extended discussion.
