Here's why:
* Meaning: It means something is finished or completed, often implying it's been done quickly or easily.
* Figurative: The literal meaning of "done and dusted" doesn't relate to the figurative meaning. You don't literally dust something to signify completion.
* Idiomatic Usage: It's a fixed expression, and you can't change the order of the words or substitute them.
Example: "The project is done and dusted, we can move on to the next one."