The person you hire to act for you doesn't have your exact incentives

Whenever one party (the principal) delegates decisions to another (the agent) — a shareholder and a CEO, a patient and a surgeon — their interests rarely align perfectly, and the agent usually knows more about their own effort than the principal can observe. That gap creates room for the agent to act in their own interest at the principal's expense, from executives padding expenses to agents pushing a quick sale over the best price. Much of corporate governance exists to narrow this gap.

— Michael C. Jensen & William H. Meckling, Theory of Agency — Journal of Financial Economics, 1976