[RussianCow]

In an ideal world, you're absolutely right. Reality is different, though—sometimes people make mistakes and generate objects on-the-fly without memoizing them, and sometimes you have to deal with third-party or legacy code that you can't control. Reasoning about the behavior of hooks in these types of situations can become really difficult, and there really is no good answer aside from being ultra-defensive with how you handle props and dependencies (which has many of its own downsides).