Disclaimer: Outline is part of world building but world-building is not just the outline. When I say world-building, I'm also talking about the outline, but if I'm talking about the outline, I'm not necessarily talking about all the things that go into world-building.
The main one would probably be killing the muse. When you write an outline, things can be forced. How? I currently do not have a clear answer.
But I noticed something interesting. When I was working on a daily basis with my editor, Francesca, we would get all hyped about something in I'm Not a Competitive Necromancer, and then the book tanked. The world building from that book was humongous. It was so big you couldn't even imagine the kind of work we laid down. It was insane. It's not a case I re-used it to basically rewrite a side-character into Joey Luciani, the MC of Casual Heroing.
And we had a good world-building, a good story, something that we were writing for ourselves. That's the keyword, the secret codeword that may unlock the 'why' outlines don't work well for many writers. For some, they do. And good for them, but I am of the opinion that Re-reading is the best form of world building.
Obviously, for a web novelist, that's a problem because you have a very limited amount of time to re-read and produce new chapters. As a traditional writer, you could have the luxury of letting things age a bit before you went over them. But as a web novel writer, you need to put out content on a weekly basis at minimum. And when that happens, having a little outline might actually be a good idea.
However, There are so many problems in writing an outline that it basically becomes impossible to think every plot-point through. I believe that plotters are actually lucky exceptions and that, by extension, most amazing works will naturally spawn from talented and experienced writers.