Digital platforms increasingly aggregate, analyze, and monetize personal data across social media, health, and mobility services to expand market reach, refine targeted advertising, and develop new services. As private regulators, platform owners define privacy settings for end users and control data access for complementors, shaping power asymmetries in personal data collection, use, and sharing. Rather than fostering mutual benefits and shared access, as inter-organizational data governance suggests, they implement practices to gatekeep on personal data. This study examines how digital platforms exercise centralized authority over personal data collection, use and sharing. Based on expert interviews with professionals with expertise related to health and mobility platforms, we reveal how structural, resource, and psychological power shape data governance practices in B2C and B2B interactions. Our findings provide first insights into personal data governance in consumer digital platforms and offer practical insights on mitigating power asymmetries.