Guy Kawasaki writes about one of the reasons:
Consensus. When it comes to doing dumb things, the sum of the parts is less than the whole. Throwing more minds at the problem means more data, more perspectives, more possible solutions, more critiques of these solutions, and more minds (and hands) implementing the solution, right?
Possibly, but theres also the downside of more people: once consensus starts to build, its harder to alter a decision. Its one thing to argue against a few people; its much more difficult to argue against the wisdom of a crowd. Individuals who hold out, question, or disagree are labeled as clueless, uncooperative, and not team players.