We don't test software systems in production either. Except we do. We should be testing in non-production environments, but non-prod is not the same as product

We don't test software systems in production either. Except we do. We should be testing in non-production environments, but non-prod is not the same as production, so it is inevitable that you will be testing a system change when you deploy to prod. If your non-prod environment is similar enough that there are NO system changes when you deploy to prod ... then your non-prod env is actually prod, because that would mean that your non-prod network routing, auth, and database are the same as prod, because changing any one of those would be a system change.