Outages happen because of code changes, sure. But they also happen because of hardware changes, some of which is possible to guard against and some isn't really. They also happen because of changes in use that expose issues that were already there but nobody noticed.

They also can happen when resource leaks add up as cleanup that used to be routine isn't done. In teams that push weekly by restarting, they may never realize they've got a (whatever) leak that will kill the servers in 6 weeks; because they probably push often enough that it's never noticed; and testing for slow leaks is hard.