Origin.
Ad account access is the governance layer of the agency relationship. When the operator owns the account and grants the agency admin access, the operator can audit the work, export the history, and revoke access on the day the relationship ends. When the agency owns the account, the operator is renting access to their own marketing data and the agency holds the export key. The asymmetry is structural and is rarely discussed at the start of the engagement because the start of the engagement is when both sides are aligned and neither party wants to introduce friction.