How is it possible for an under-contract author to work on just one project at a time?

In the world of literary fiction, it's extremely unusual for a writer to have more than one thing going at once. It seems like people write books, sell them, revise them, see them published, and then there's a wait of 3-4 years before the next one. Other genres have other timescales. In YA, lots of people seem to be putting out a book every year. And in…