Why do professional golfers have their caddie carry the bag instead of using a buggy?

Carts are not aloud on the PGA tour. I can only think of one exception to the rule. Due to a disability, Casey Martin, was unable to walk a full 18 holes. The PGA gave him special permission to use a cart. The only stipulations were that neither his caddy nor none of his equipment were allowed in or on the cart. The other reason that golfers use caddies is that a caddy provides them with information about the course, the greens, and other aspects of the game along with making suggestions.