Simple. On the day Article 50 was invoked, a No-Deal Brexit was possible. So preparations should have started. They've had nearly 2 years.
However, Whitehall, and a sizeable proportion of the Cabinet were (and still are) determined that Brexit should be as soft as possible, or not happen at all. Remember, Cameron expressedly forbade any such preparations whilst PM. So little or nothgin has been done.
This is a kind of reverse brinksmanship. Instead of convincing the other side you can walk away, you convince the rest of your side that you cannot.