I was going to say "because it's convenient", but I realized that you're right: if it's identical anyway, the release date is as good a timestamp of the code as you're going to get. If for some sysadmin reason the build time is relevant, file metadata can tell you.
Maybe I want to know that someone was messing with stuff, and even if they put something 'back in place' that matches, I may still want to know that it was built this morning and that something is going on.
