On page 289 (in the paperback edition), Diamond gives us an answer to this question. He claims that societies never merge voluntarily. He says that there are only two ways that mergers occur. They occur "under the threat of external force" or they occur when one society conquers another.
The second of these is self-explanatory but the first is less so. Diamond argues that there are times when two or more societies merge together to help each other be better able to resist aggression by some outside force. Diamond gives the example of the Chreokee Indians. This tribe was originally divided into 30 or 40 independent groups but those groups merged in an attempt to gain more power and be able to resist the white settlers who threatened them.
Thus, Diamond argues, societies merge involuntarily. They can merge through conquest or they can merge in an attempt to defend themselves against a powerful foe.