It comes to supports can't carry vs supports can't win games.
Supports can totally win games through the utility they bring. However, a support will never have the carry potential of say, a Vayne who is playing just as well.
This is because Vayne brings damage that supports do not as well as better objective taking.
Supports, while lacking the ability to carry w/o much team support (in comparison to other roles) can win games because supports offer more power when working with other champs than other roles do. Except maybe in the case of a few jglers.