Carries are pawns. There's no nice way to put it.
What do you do as a carry? You farm, you farm, you buy items, and you ball out of control.
What does a support do? I support wins the fucking game. All strategic value in a game is on the backs of the support. Where should my carry be farming? Should I stack and pull, or would that risk my carry too much? Where should I ward? Where do I want the next team fight to happen? What towers should we be focusing? Who should I gank? Who should I put pressure on? Should I buy smoke, dust, sentries, wards, or a bottle for mid?
Carries can't carry if they don't get good farm. There's a very low skill cap for carrying. Once you get to a certain level of skill with carrying, your win depends on your farm vs. their farm, not how well you play. And that farm, that early game, is in the hands of the three or four supports who will be playing a game before gold advantage becomes a predominant factor. They are playing the game when it is skill vs skill, not economy vs. economy.
Supports. Make. Shit. Happen.