Because it actually catches issues. Your unit tests are never good enough. Your integration tests won't match your production environment. Your canaries only t

Because it actually catches issues. Your unit tests are never good enough. Your integration tests won't match your production environment. Your canaries only test the happy path. If your goal is full CI/CD, some changes will make it through that will impact at least a subset of your customers in production without being caught. A good code review process utilizes a larger portion of the team's understanding of a system, not just your own, to help catch some of these issues. This understanding could be system, product, or inter-team dependencies that you will never fully codify into an automated process. It also socializes best practices, help folks learn new patterns and improve as software engineers.