Ending the Silos
Many organizations are dark, as you describe it, because of the silos you mention. Now recognizing the value, costs, and legal protections consolidation create, many organizations are slowly but surely pulling together their data repositories. It's challenging, but the payoffs -- as those who have accomplished the task often can attest to -- are many and rich.
On the consumer side, I'm sure we can all recall instances where our data is housed multiple times within a business. Often, that results in multiple emails/calls/letters, sometimes using different information. Multiply that across millions of people and that saving alone adds up. On the legal front, not knowing what you have (and, therefore, being unable to correctly secure it at times) is a hazard for many industries.