Why are our responsibilities and duties centralized instead of more evenly distributed? I understand that academia is a very competitive place where motivated people are always trying to get ahead of the others. What I have noticed is that few famous/known/recognized people tend to commit to more and more duties, some of them unpaid, to the point that they have very limited time to perform  some of these duties.

For example head of departments are also main editors of famous journals, may be a member of an international panel, on top of all the other duties. Then what I read and hear is that we are overworked, but it seems that many duties which bring prestige but involve a fair amount of work are centralized. 

Because people tend to say "yes" then why is not all this work more evenly distributed? It seems that this would benefit more people and make the system less cluttered. I am referring for example to journal editors, or having 50 phd/post-docs and not being able to properly supervise any of them, etc.
One way to answer this question is to borrow a concept from economics known as the Pareto Principle. This principle, slightly reworded, states that 80% of the work is done by 20% of the people. This concept plays out in many different fields beyond your question in relation to academics. The talented people tend to do produce way more than the average and not so talented.This gives the appearance of centralization when it is really just the 20% producing 80% of the results