Conversation in Games: Trigger, Branch, Repeat

Bento Smile's Air Pressure

Why is there so little variety in conversation systems in games?  Conversation is one of the most essential and frequent things humans do.  It is a common source of interest, drama, and comedy in both real life and fiction.  It reveals character, it advances plotlines, and it uncovers information.  It's hugely important to most stories about people.  Yet almost every videogame, regardless of genre, that employs playable conversation uses the same two basic mechanics for it.  Some use one or the other, some use a combination.  These two mechanics are:

Response Triggering.  The player employs some kind of trigger action towards another character.  The trigger could be an object, a topic selected from a list, or just the choice to talk to the character at all.  Each trigger leads to a specific canned response from the character.  Employing the same trigger multiple times will usually get the same response.  The order in which triggers are deployed is not significant.

Branching Paths.  This is a variation on basic response triggering in which each response opens up a new set of triggers.  This allows for conversations with a temporal progression and usually involves choices about the direction the dialogue takes.