Why can't you generate a use case diagram from a state diagram?

Question

Why can't you, for example, take a state diagram, and make use case diagram based on this state diagram?

Answer 1

No, you can't. The reason is simple:

• Use-cases are about actors. Actors are outside the system, whereas the state diagram is about the system.
• Use-cases should be about goals of the actors. You cannot infer intentions in the head of people from what happens in the system. If I write a word CAPITALIZED, you cannot for sure tell if it is because my caps-lock is blocked, because I want to draw your attention to something, or because I give another meaning to capitalized words.

Since most of the information related to use-cases are missing, you cannot derive them from the state-diagram, except if you would have additional contextual information. Even then, it would be an educated guess, more than a rational, objective and repeatable process.