My hypothesis is that the reason why consensus-based open source projects exist but consensus-based states don’t, is that if you don’t like a project, you can quit. And the default state is not to be in any project. Unfortunately, you can’t quit a state without joining another, and quitting a state is very expensive (in many ways). Also, you have to abandon everything if you want to quit it. You can’t just fork the US and have the better version win. That would be nice, but unfortunately impossible.
In a complex system like a state, every decision is controversial and therefore requires a vote.