The commitment and interdependence of the employment relationship smooths over most disagreements at work. People may not like it, but they stay aboard because, well, what are they going to do, leave? And what's the company going to do, fire them?
No such arrangement exists in voluntary efforts. People will work together fine as long as they all agree on most things. Once a real conflict emerges, without the underlying commitment and interdependence, people just decide it's not worth the frustration and walk away.