I have often argued that the founders of Agile did not provide reasons why their approaches worked just that they did. Their was empirical evidence, proven by doing the work, or, as they state in the beginning of the manifesto - uncovering better ways of developing software by doing it and helping others do it. From their very pragmatic approach, they figured out that better software was created by following the values and principles. One of those discoveries was that better software was created by self-organizing teams.