> A few projects have a clear set of members.

Exactly.

I'm not too versed in political theory, but one thing from my perception that provides coherence/continuity in nations is the immobility of participants (for better or worse of the individual). E.g. if you have 70/30 election outcome you still have to factor in the needs of the 30% as they may provide important economic functions and can't just leave on election loss.

In contrast, in an open source project, even if you can clearly delineate membership and based on that voting rights for a democratic process there is very little preventing the "losers" from forking (it almost entirely comes down to brand). The outcome of that would just be an empty brand shell with a good chunk of their contribution activity gone.