Those of us who spend time with collections (not just in museums) know that several pieces will never wake from hibernation. While candidates for deaccessioning, removing objects could create a major image problem and also requires a large amount of work for staff. Safely storing and housing these objects similarly requires massive amounts of resources and represents a safety risk as illustrated by the British Museum.
The Storytellers
I have a more likely theory (maybe I should actually ask a curator why): they get recruited to those positions because they are good storytellers. They craft and relate narratives around institutions, around objects, around exhibitions that inspire curiosity and devotion among donors and earn loyalty from those around them.