It's easier, in some respects, to outline than to write the story.
If only because it moves much more swiftly. Inspiration can stiffen up in writing it out as well as in outlining, and outlining, for most of it, gives an easy chance to stuff all sorts of bright-eyed plot bunnies in here and in there, because the structure is still malleable -- in writing it out, any change is going to require alterations, not just devising new things in the future. And then there's the brute fact of knocking out the prose. It's much easier to write that when Murgatroy saw something down at the river, half the party left to investigate, while the rest stayed investigating a way into the castle than to write the scene. And then there's the endless revision. . . hopefully not endless.
Because while outlining is the easiest for me, there have been times when I've been caught endlessly writing, or endlessly revising. All of which have to be done in proportion. If done in perfect proportion, the stories would finish in due course and get hurled over the wall to the waiting markets. But as it stands, even sitting in the chair, fingers on the keyboard or pen in hand, is no guarantee that you're not vacuuming the cat.