Passing tests is useful evidence, not a guarantee that a change is safe to merge. Tests check specific behaviors under specific conditions, so they can miss unexpected inputs, flawed assumptions, or interactions with their parts of the codebase.
Code review helps investigate these gaps. Reviewers examine whether a patch solves the underlying problem, what else it might affect, and whether the tests would catch an incorrect implementation.
In Bitcoin development, these questions are part of protecting the software people rely on.