It's not. Imagine there is a bug that stops the app from starting. It could be anything, from a configuration error (e.g. against the database) to a problem with warmup (if necessary) or any kind of other bug like an exception that only triggers in production for whatever reasons.

EDIT: and worse, it could be something that just started and would even happen when trying to deploy the old version of the code. Imagine a database configuration change that allows the old connections to stay open until they are closed but prevents new connections from being created. In that case, even an automatic roll back to the previous code version would not resolve the downtime. This is not theory, I had those cases quite a few times in my career.