Maybe a better phrasing is that when an employee leaves unexpectedly, it's a failure of management. Which it is, because the very purpose of management is to keep things managed.

So you can either pretend that employees will never leave and try to make that as true as possible, which is a poor way to manage the situation (but better than not paying attention at all), or you can try to figure out which employees are likely to leave and what you should do about it.