Why might a company choose to promote internally when filling a vacancy, rather than advertising publicly?

Filling a vacancy with an existing, known employee is quicker and involves less onboarding risk

Companies often favor internal promotion as a practical strategy for filling open positions because it significantly reduces risk and speeds up the time required to fill the role. By promoting an existing employee, the company already has established knowledge regarding that individual’s performance, work ethic, and cultural fit, thereby lowering the onboarding risk associated with hiring an unknown external candidate. Furthermore, the logistical process of integrating an internal candidate is usually much faster than initiating a comprehensive external search, screening hundreds of applications, and completing extensive onboarding procedures for someone new to the organization's systems and culture.