Probably from the fact that disabling these email addresses would be a distinctly user-hostile move. If Apple determines that the developer is actually a malicious entity who slipped malware past the App Review process, then it might be reasonable for them to disable the forwarded emails as well as a malware vendor has no legitimate reason to talk to "users", but beyond that specific scenario, cutting off the email address harms the users who signed up for that service. For example, if the app actually represents a web service, and Apple determines that the app shouldn't be in the App Store for whatever reason, blocking "Sign In With Apple" (which is distinct from removing the app from the app store) and cutting off the email address means the user has no way to recover their account even though the service is still active, just without an app. There is no reason for Apple to take this step as it benefits nobody and only serves to harm Apple user.