People do work together, and when they do you get opensouce projects like AROS and MiniMig 
Problem for even open source is that it has to be a modern open source to survive. I think that is the biggest loadstone for the Amiga community, it's just reimplementation of an old school OS (which was cutting edge at the time). Until that loadstone is gone, any cooperation is going to be futile. I know, I just touched a third rail, but once the fog of battle clears and the view of the destruction that the community has gone under for the past 17 years, there is not much left worth fixing, it's time to rebuild. That means the old has to be torn down for the new.