Some of it is fun stuff vs. heavy lifting. There is the New, Bright, Sparkly Ideas side of it. In the outline, you are still playing around with them, putting them next to each other and seeing if they get along rather than fitting them into place. But -- there are New Bright Sparkly Ideas that show up in the middle of writing, too. Getting into the story down at a detail level brings up sharp little details that you didn't notice at an outline level. And you can go for broke on the weird, wacky wonderful things that you just jot down in the novel. And the outline has its share of heavy lifting, too. Figuring out cause and effect to fit into the character arc and the rising action of the plot until it all collides.
I think it's partly the length. Outlines have to be expanded into several pages for each line. And then they have to be revised -- and in my experience, the amount of time to revise expand geometrically with the length of the story. It's a lot easier to get the end of the story.
Which means that the writer has to spend more time writing and revision than outlining, no matter how many resplendent story ideas come along.