In an organization where everyone is equal, adding a new person to the org doesn’t benefit the people who are already in it. Let’s say you have a coop bagel shop- is there any particular reason to want to cut your own paychecks until you have saved up enough to open another one? Or let’s say you live in an egalitarian, peaceful country. Is there any reason to want to invade your neighbors and annex territory? No.
But, if you put one person in charge, then suddenly you have a person in a position of authority with a vested interest in expanding the franchise, since he gets to skim off the labor of everyone underneath him. In a stroke, you’ve invented empire, war, and capitalism. Come back in a hundred years and the organizations that expanded the most are also the most heirarchical.
I don’t think this is a good thing, by the way, just a historical factor to be aware of.