The value of what is already known changes quickly. Some of what is known is obsolete and toxic. Applying someone else's knowledge often has no glory. Knowledge is often not in the heads of the people who need it. Knowledge is learned, and there's no time-out for learning.
Different people learn differently.
Knowledge is not naturally mobile within an organization. Large organizations are culturally diverse. Large organizations are geographically dispersed. KM and collaborative web tools are in their infancy. What to know and when to know it is a vital strategic issue.