When I update an app on my phone, if it fails or changes considerably, it only affects me. When I update anything on a server(s), it's going to affect hundreds, thousands, or even millions of other people and getting back to the state it was in before the update can cost hundreds of hours and thousands of dollars, not to mention potential money lost from the affected users.
Exactly the same can happen by rolling out a buggy app to millions of other people. You can "brick" their app, and then have to work out how to get the fix rolled out to every user.
When I update an app on my phone, if it fails or changes considerably, it only affects me. When I update anything on a server(s), it's going to affect hundreds, thousands, or even millions of other people and getting back to the state it was in before the update can cost hundreds of hours and thousands of dollars, not to mention potential money lost from the affected users.