Why not self-management?
Organizations successfully practicing self-management are the exception, not the norm. The norm, unfortunately, is organizational dysfunction despite hierarchical management. A few examples:
Most organizations are better off but its leaders still lack the kind of integrity and maturity needed for successful self-management. Even modern organizations that have tried and failed at some variation of self-management.
That said, Cleararchy is not incompatible with self-management. A given outcome owner may choose to use a self-management approach with their team. An organization cannot simply pursue a goal of self-management amongst all its teams — self-management has to ensue. At the same time, we cannot put desired outcomes at risk waiting for self-management to happen. Hence Cleararchy.