Testing is a best-case scenario to validate the system's correctness. But, it doesn't predict the failure cases that may occur in production. Experienced engineering teams would tell you that production environments are not uniform and full of exciting deviations.
The fun fact is – testing in production helps you test the code changes on live user traffic, catch the bugs early, and deliver a robust solution that increases customer satisfaction. But, it doesn't help you detect the root cause of the failure.